P0422 Main Catalyst Efficiency Below Threshold (Bank 1)

The code P0422 means that the efficiency of the main catalyst (Bank 1) is below the required threshold according to the manufacturer’s specifications.

🔍 Decoding the P0422 Code

Generic OBD-II code: It applies to all OBD-II compatible vehicles, but the causes/repairs may vary by brand.
Inefficient catalyst (Bank 1): The PCM (Powertrain Control Module) detected that the gases exiting the catalyst are too similar to those entering, meaning the catalyst is no longer effectively converting pollutants (HC, CO, NOx).
Bank 1 = The side of the engine where cylinder #1 is located.

⚠️ Symptoms of the P0422 Code

Check Engine light illuminated,
Increased pollutant emissions,
In some cases: higher fuel consumption or reduced engine performance,
Failure of the emissions test (technical inspection).

🔧 Possible Causes

Faulty or worn-out catalyst (very common),
Faulty or misread downstream oxygen sensor (after catalyst),
Abnormal air/fuel ratio due to:
- Injector issues,
- Airflow meter (MAF) problem,
- Air intake leak,
- Poor ignition,
- Bad condition of spark plugs, filters, or coils,
Use of a low-quality replacement catalyst (very common on Toyota, Subaru, etc.),
Exhaust leak before or around the catalyst.

🛠️ Possible Solutions

1. Diagnose the Problem at the Source

OBD-II scan: confirm if the downstream oxygen sensor is switching too quickly (almost like the upstream one),
Check for exhaust leaks before the catalyst,
Check the condition and response of the oxygen sensors.

2. Check Engine Condition

Ensure the engine is running optimally: air/fuel mixture, ignition, compression, etc.
Perform a tune-up: spark plugs, coils, injectors, filters.

3. Catalyst Replacement

If the catalyst is indeed faulty:
- Replace with an original (OEM) or certified catalyst (especially for Toyota/Subaru),
- Avoid “universal” or cheap models if your vehicle is sensitive.

✅ Conclusion

The P0422 code indicates that your catalyst is no longer doing its job correctly. Before replacing it, ensure the engine is running well, otherwise a new catalyst may fail quickly.

P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)

P0423 Catalyst Efficiency The computer in your vehicle (the PCM) has detected that the catalyst (or “catalytic converter”) on bank 1 (the side of the engine where cylinder #1 is located) is not operating efficiently enough to reduce polluting emissions.

Role of the Catalytic Converter

It is a part of the exhaust system whose mission is to transform polluting gases (carbon monoxide, unburned hydrocarbons) into non-toxic gases (carbon dioxide, water vapor) using precious metals (like platinum) and heat.
To do its job, it must reach a very high temperature (at least 800 °F or 427 °C).

How Does the Computer Detect It?

The computer uses two oxygen sensors (lambda sensors):

An upstream sensor (before the catalyst) that measures the gases exiting the engine.
A downstream sensor (after the catalyst) that measures the gases after they have passed through the catalyst.

If the catalyst is working WELL: It “cleans” the gases. The downstream sensor sends a signal that is stable and very different from that of the upstream sensor.
If the catalyst is working POORLY: It no longer transforms the gases. The signals from both sensors become too similar. The computer deduces that the catalyst is inefficient and triggers the P0423 code and illuminates the check engine light.

Severity and Symptoms

Severity: Serious. This code should not be ignored, as it indicates a pollution problem and can mask other engine issues.
Possible Symptoms:
- 🚨 Check engine light illuminated (the most common symptom).
- ⛽ Increased fuel consumption.
- 🐌 Loss of engine power.
- Sometimes, no noticeable symptoms other than the light.

Possible Causes (in order of likelihood)

The causes are not limited to the catalyst itself:

Engine Problems: Unresolved misfires send too much unburned fuel into the exhaust, which overheats and destroys the catalyst. This is often the root cause!
Exhaust Leaks: A leak before the catalyst (manifold, pipe) lets in cold air, preventing it from reaching its operating temperature.
Faulty Oxygen Sensors: A downstream sensor sending a bad signal can trick the computer. (This is more common than a dead catalyst).
Air Intake Problems: A faulty mass air flow (MAF) sensor or manifold absolute pressure (MAP) sensor can disrupt the air/fuel mixture and damage the catalyst.
Wiring: Damaged or corroded electrical wires going to the oxygen sensors.
The Catalyst Itself: As a last resort after eliminating all other causes. It could be clogged, melted, or “poisoned” (by engine oil or an unsuitable fuel additive).

What To Do? Diagnostic Procedure

Warning: Diagnosis requires an automotive scanner, a multimeter, and mechanical knowledge.

Read All Codes: If there are other codes (especially for misfires, O2 sensor, MAF, MAP), repair them first. Clear the codes and perform a road test to see if the P0423 returns.
Visual Inspection:
- Check for exhaust leaks (hissing sounds, black soot) before the catalyst.
- Inspect the wiring and connectors of the oxygen sensors for burns or damage.
Use a Scanner:
- Observe the live data from both oxygen sensors.
- If the signals from both sensors are identical or very similar once the engine is warm, the catalyst is most likely dead.
- If the downstream sensor is slow to react or its signal is flat, it might be faulty.
Test the Oxygen Sensors with a multimeter if possible.

Conclusion: Don’t rush to replace the catalyst! It is a very expensive part. In the majority of cases, the P0423 code is a consequence of another problem (misfires, leak, sensor). Always start by inspecting and repairing the other possible causes.

If everything else is eliminated, then replacing the catalyst will be necessary. Prefer a new, quality catalytic converter over a refurbished one, which is often less reliable.

P0424 Heated Catalyst Temperature Below Threshold (Bank 1)

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Dodge, Ram, Ford, GMC, Chevrolet, VW, Audi, Toyota, etc.). Although generic, specific repair steps may vary by make/model.

When I diagnosed a P0424 code, I found that the Powertrain Control Module (PCM) detected a problem with the heated catalytic converter temperature for engine bank one. Bank 1 indicates the problem occurred in the engine bank containing cylinder number one.

While I cannot state with absolute certainty that a P0424 code only applies to diesel engine vehicles; I have never seen it exposed in anything other than diesel engine vehicles.

Heated catalytic converters are used to reduce exhaust emissions produced by diesel engines. A filtration block, composed of ceramic fibers interwoven with platinum and other precious metals, enclosed in an inline steel housing and placed in the exhaust system, is the key component of the heated catalytic converter. Beyond the heat created as a byproduct of engine exhaust, diesel engine exhaust frequently needs to be heated further to sufficiently reduce nitrogen oxide particles (N2O), carbon monoxide, and unburned hydrocarbons into harmless ions of nitrogen, oxygen, carbon dioxide, and water. Temperatures inside the heated catalytic converter are often designed to reach a minimum of 1,200 degrees Fahrenheit. To produce the extreme temperatures needed to reduce emissions from diesel engines, diesel exhaust fluid is injected into the heated catalyst at intervals controlled by the PCM.

Upstream and downstream exhaust temperature sensors are used by the PCM to monitor the heated catalytic converter threshold. This is necessary to ensure the filtration element (catalyst) is sufficiently heated.

Exhaust gases are pushed through the manifold, into the exhaust pipe, past the upstream exhaust temperature sensor, and through the heated catalytic converter. After passing through the heated catalytic converter, they pass the downstream exhaust temperature sensor. The PCM is programmed with specific minimum requirements regarding the temperature differential between exhaust gases entering the catalyst and exhaust gases exiting the catalyst. These programmed requirements are referred to here as the heated catalyst temperature threshold.

If the heated catalyst temperature variation (between the upstream and downstream exhaust temperature sensors) is below the minimum allowed threshold, a P0424 code will be stored and a Malfunction Indicator Lamp (MIL) may be illuminated.

Other Bank 1 catalyst malfunction codes include P0420, P0421, P0422, P0423, P0425, P0426, P0427, P0428, and P0429.

Code Severity and Symptoms

If a P0424 code has been stored in your vehicle, it may indicate that the diesel exhaust fluid injection system is not functioning. This code should be classified as severe.

Symptoms of this code may include:

Decreased fuel efficiency
Lack of overall engine performance
Excessive black smoke from the exhaust
Other related diagnostic codes
MIL (Malfunction Indicator Lamp) illumination

Causes

Possible causes for this code being set are as follows:

Faulty diesel exhaust fluid injection system
Inadequate diesel exhaust fluid
Faulty exhaust temperature sensor(s)
Burned, broken, or disconnected wiring and/or connectors

Diagnostic and Repair Procedures

To diagnose a P0424 code, I would need a diagnostic scanner, a digital volt/ohmmeter (DVOM), an infrared thermometer (preferably with a laser pointer), and a reliable vehicle information source (such as All Data DIY).

Before attempting to diagnose a stored P0424 code, all codes related to the diesel exhaust fluid injection system should be addressed, and the engine must be running efficiently.

I normally start with a visual inspection of the system’s wiring harnesses and connectors, focusing on harnesses routed near hot exhaust pipes and manifolds, as well as those near sharp objects like exhaust shields.

Next, I would connect the scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes and freeze frame data. I’ve found that writing down this information can prove helpful later in my diagnosis. Now, I would clear the codes and test drive the vehicle to see if the P0424 immediately resets.

I would start the engine and allow it to reach normal operating temperature if the P0424 is reset. I would test drive the vehicle while carefully using the scanner’s data stream to observe the exhaust temperature sensor input data. Limiting the data stream to include only relevant data will provide a more accurate data sample.

Once the PCM enters closed-loop operation and driving conditions are met for the diesel exhaust fluid injection system to be activated, the exhaust gas temperature sensor signals should reach an acceptable threshold and remain relatively stable. If not, access the catalytic converter by placing the vehicle on a lift or hoist and use the infrared thermometer to take a manual temperature reading of the exhaust pipe in the immediate vicinity of the exhaust temperature sensor(s). Compare the temperature displayed on the infrared thermometer with the data presented on the scanner to determine if there is a problem with either exhaust temperature sensor.

Consult your vehicle information source for the vehicle’s recommended specifications and use the DVOM to check the resistance of the exhaust temperature sensor in question.

Additional diagnostic notes:

The presence of a stored P0424 does not often condemn the catalytic converter
Exhaust temperature sensor failure is more common than catalytic converter failure
Diesel exhaust fluid must be present and of the correct type before diagnosing a P0424
Disconnect all associated controllers before attempting to test system circuit resistance with the DVOM

P0425 Catalyst Temperature Sensor Circuit Malfunction (Bank 1, Sensor 1)

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have a catalytic converter temperature sensor (Subaru, Ford, Chevy, Jeep, Nissan, Mercedes-Benz, Toyota, Dodge, etc.). Although generic, the specific repair steps may vary by make/model.

The catalytic converter is one of the most important emission control devices on a vehicle. Exhaust gases pass through the catalytic converter where a chemical reaction occurs. This reaction transforms carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into harmless water (H2O) and carbon dioxide (CO2).

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the signals from the oxygen sensor (O2), the Powertrain Control Module (PCM) can determine if the catalytic converter is functioning correctly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volt indicates a lean air/fuel mixture, while 0.9 volt indicates a rich mixture. If the converter is working properly, the downstream sensor should read steadily around 0.45 volts.

The efficiency and temperature of the catalytic converter go hand in hand. If the converter is working as it should, the outlet temperature should be slightly higher than the inlet. The old rule of thumb was a 100-degree Fahrenheit difference. However, many modern vehicles might not show that much difference.

There is no actual “catalytic converter temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “Bank 1” part of the code indicates the issue is with the engine’s first bank. That is, the bank which includes cylinder number 1. “Sensor 1” refers to the sensor mounted upstream of the catalytic converter.

Trouble code P0425 is set when the PCM detects a malfunction in the Bank 1 Sensor 1 catalytic temperature sensor circuit.

Related diagnostic trouble codes include:

P0426 Catalytic Converter Temperature Sensor Circuit Range/Performance (Bank 1, Sensor 1)
P0427 Catalytic Converter Temperature Sensor Circuit Low (Bank 1, Sensor 1)
P0428 Catalytic Converter Temperature Sensor Circuit High (Bank 1, Sensor 1)

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P0425 engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Causes

Possible

causes of this P0425 code include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / Faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the upstream oxygen sensor and its associated wiring. Look for loose connections, damaged wiring, etc. Also check for exhaust leaks visually and audibly. An exhaust leak can cause a false oxygen sensor code. If damage is found, repair as needed, clear the code, and see if it returns.

Next, check for Technical Service Bulletins (TSBs) regarding the issue. If nothing is found, you will need to proceed with step-by-step diagnosis of the system. The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to a diagnostic flowchart specific to your vehicle’s make/model.
Check for Other DTCs

Oxygen sensor codes can often be set due to engine performance issues causing an unbalanced air/fuel mixture. If other trouble codes are stored, you should address those first before proceeding with oxygen sensor diagnosis.
Check Sensor Operation

To do this, it’s best to use a scan tool or, better yet, an oscilloscope. Since most individuals don’t have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn the scan tool on and choose the parameter for Bank 1 Sensor 1 voltage from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode. The sensor should switch rapidly between rich and lean (0.1 volts and 0.9 volts). If the sensor’s response is slow, it is likely faulty and needs replacement.

If the sensor reads above 0.55 volts consistently, either the sensor is faulty, the air/fuel mixture is too rich, or there is an open in the sensor’s signal circuit. If the sensor reads below 0.35 volts consistently, either the sensor is faulty, the air/fuel mixture is too lean, or there is high resistance or a short in the signal wire to the PCM.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before continuing, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles and ALLDATADIY offers single-vehicle subscriptions. To check for continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Next, you will want to check the ground side of the circuit. To do this, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads Out of Limits (OL), there is an open circuit on the ground side of the circuit that will need to be located and repaired. If the meter reads a numerical value, there is continuity to ground.

Finally, you will want to verify that the PCM is processing the O2 sensor signal correctly. To do this, leave all connectors connected and insert a back-probing pin into the PCM’s signal terminal. Set the digital multimeter to the DC volts setting. With the engine warmed up, compare the voltage reading on the meter to that on the scan tool. If the two do not match, the PCM is likely faulty or requires reprogramming.

P0426 Catalyst Temperature Sensor Circuit Range Performance (Bank 1, Sensor 1)

What does it mean?

The catalytic converter is one of the most important emission devices on a vehicle. Exhaust gases pass through the catalytic converter where a chemical reaction occurs. This reaction transforms carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into harmless water (H2O) and carbon dioxide (CO2).

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the oxygen sensor (O2) signals, the powertrain control module (PCM) can determine whether the catalytic converter is functioning properly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volt indicates a lean air/fuel mixture, while 0.9 volt indicates a rich mixture. If the converter is working correctly, the downstream sensor should read steadily around 0.45 volts.

The catalytic converter’s efficiency and temperature go hand in hand. If the converter is working as it should, the outlet temperature should be slightly higher than the inlet. The old rule of thumb was a 100-degree Fahrenheit difference. However, many modern vehicles might not show that much difference.

There is no actual “catalytic converter temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “Bank 1” part of the code indicates the problem is with the engine’s first bank. That is, the bank that includes cylinder number 1. “Sensor 1” refers to the sensor mounted upstream of the catalytic converter.

Trouble code P0426 is set when the PCM detects a malfunction with the catalytic converter temperature sensor.

Related diagnostic trouble codes include:

P0425 Catalytic converter temperature sensor circuit malfunction (Bank 1, Sensor 1)
P0427 Catalytic converter temperature sensor circuit low (Bank 1, Sensor 1)
P0428 Catalytic converter temperature sensor circuit high (Bank 1, Sensor 1)

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P0426 engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Causes

Possible causes for this P0426 code include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the upstream oxygen sensor and its corresponding wiring. Look for loose connections, damaged wiring, etc. Also check for exhaust leaks visually and audibly. An exhaust leak can cause a false oxygen sensor code. If damage is found, repair as needed, clear the code, and see if it returns.

Next, check for Technical Service Bulletins (TSBs) regarding the issue. If nothing is found, you will need to move on to step-by-step diagnosis of the system. The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you will need to refer to a diagnostic flowchart specific to the vehicle’s make/model.
Check for Other DTCs

Oxygen sensor codes can often be set due to engine performance problems causing an unbalanced air/fuel mixture. If other trouble codes are stored, you should address them first before proceeding with oxygen sensor diagnosis.
Check Sensor Operation

To do this, it’s best to use a scan tool or, even better, an oscilloscope. Since most individuals don’t have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn the scan tool on and choose the Bank 1 Sensor 1 voltage parameter from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode. The sensor should switch rapidly between rich and lean (0.1 volts and 0.9 volts). If the sensor’s response is slow, it is likely faulty and needs replacement.

If the sensor reads above 0.55 volts constantly, either the sensor is faulty, the air/fuel mixture is too rich, or there is an open in the sensor’s signal circuit. If the sensor reads below 0.35 volts constantly, either the sensor is faulty, the air/fuel mixture is too lean, or there is high resistance or a short in the signal wire to the PCM.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before proceeding, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles and ALLDATADIY offers single vehicle subscriptions. To check continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor which will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Next, you will want to check the ground side of the circuit. To do this, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads Out of Limits (OL), there is an open circuit on the ground side of the circuit which will need to be located and repaired. If the meter reads a numerical value, there is continuity to ground.

Finally, you will want to verify that the PCM is processing the O2 sensor signal correctly. To do this, leave all connectors connected and insert a back-probing pin into the PCM’s signal terminal. Set the digital multimeter to the DC volts setting. With the engine warmed up, compare the voltage reading on the meter to the one on the scan tool. If the two do not match, the PCM is likely faulty or requires reprogramming.

Low Catalyst Temperature Sensor Circuit (Bank 1, Sensor 1)

What Does It Mean?

This diagnostic code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have a catalytic converter temperature sensor (Subaru, Ford, Chevy, Jeep, Nissan, Mercedes-Benz, Toyota, Dodge, etc.). Although generic, the exact repair steps may vary by make/model.

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the oxygen sensor (O2) signals, the Powertrain Control Module (PCM) can determine if the catalytic converter is functioning correctly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volt indicates a lean air/fuel mixture, while 0.9 volt indicates a rich mixture. If the converter is working properly, the downstream sensor should read steadily around 0.45 volts.

Catalytic converter efficiency and temperature go hand in hand. If the converter is working as it should, the outlet temperature should be slightly higher than the inlet. The old rule of thumb was a 100-degree Fahrenheit difference. However, many modern vehicles might not show that much difference.

There is no actual “catalyst temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “Bank 1” part of this code indicates the problem is coming from the engine’s first bank. That is, the bank that includes cylinder number 1. “Sensor 1” refers to the sensor mounted upstream of the catalytic converter.

Trouble code P0427 is set when the PCM detects a low catalyst temperature sensor signal. This typically indicates the circuit is shorted.

Related diagnostic codes include:

P0425 Catalyst Temperature Sensor Circuit Malfunction (Bank 1, Sensor 1)
P0426 Catalyst Temperature Sensor Circuit Range/Performance (Bank 1, Sensor 1)
P0428 Catalyst Temperature Sensor Circuit High (Bank 1, Sensor 1)

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P0427 engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Causes

Possible causes of this P0427 code include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / Faulty PCM

Diagnostic and Repair Procedures

To do this, it is best to use a scan tool or, even better, an oscilloscope. Since most individuals do not have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn on the scan tool and choose the Bank 1 Sensor 1 voltage parameter from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode. The sensor should rapidly switch between rich and lean (0.1 volts and 0.9 volts). If the sensor’s response is slow, it is likely faulty and needs replacement.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before continuing, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles and ALLDATADIY offers single-vehicle subscriptions. To check continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor which will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Next, you will want to check the ground side of the circuit. To do this, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads Out of Limits (OL), there is an open circuit on the ground side of the circuit which will need to be located and repaired. If the meter reads a numerical value, there is continuity to ground.

P0428 High Catalyst Temperature Sensor Circuit (Bank 1, Sensor 1)

What does it mean?

This diagnostic code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles that have a catalytic temperature sensor (Subaru, Ford, Chevy, Jeep, Nissan, Mercedes-Benz, Toyota, Dodge, etc.). Although generic, the exact repair steps may vary by make/model.

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the oxygen sensor (O2) signals, the Powertrain Control Module (PCM) can determine if the catalytic converter is functioning correctly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volts indicates a lean air/fuel mixture, while 0.9 volts indicates a rich mixture. If the converter is working properly, the downstream sensor should stabilize regularly around 0.45 volts.

There is no actual “catalyst temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “bank 1” part of the code indicates that the problem is with the engine’s first bank. That is, the bank that includes cylinder number 1. “Sensor 1” refers to the sensor mounted upstream of the catalytic converter.

Trouble code P0428 is set when the PCM detects a catalyst temperature sensor signal. This typically indicates an open circuit.

Related diagnostic codes include:

P0425 Catalyst Temperature Sensor Circuit Malfunction (Bank 1, Sensor 1)
P0426 Catalyst Temperature Sensor Circuit Range/Performance (Bank 1, Sensor 1)
P0427 Catalyst Temperature Sensor Circuit Low (Bank 1, Sensor 1)

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P0428 engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Causes

Possible causes of this P0428 code include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / Faulty PCM

Diagnostic and Repair Procedures

Oxygen sensor codes can often be set due to engine performance issues causing an unbalanced air/fuel mixture. If other trouble codes are stored, you should address them first before proceeding with oxygen sensor diagnosis.
Check Sensor Operation

To do this, it’s best to use a scan tool or, even better, an oscilloscope. Since most individuals do not have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn on the scan tool and choose the Bank 1 Sensor 1 voltage parameter from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode. The sensor should switch rapidly between rich and lean (0.1 volts and 0.9 volts). If the sensor’s response is slow, it is likely faulty and needs replacement.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before continuing, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles and ALLDATADIY offers single-vehicle subscriptions. To check continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Finally, you will want to verify that the PCM is processing the O2 sensor signal correctly. To do this, leave all connectors connected and insert a back-probe pin into the PCM’s signal terminal. Set the digital multimeter to the DC volts setting. With the engine warmed up, compare the voltage reading on the meter to that on the scan tool. If the two do not match, the PCM is likely faulty or requires reprogramming.

Catalytic Converter Heater Control Circuit (Bank 1)

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Toyota, Jeep, Honda, Nissan, Subaru, GM, Ford, etc.). Although generic, specific repair steps may vary by make/model.

If you’re facing diagnosis of a stored P0429 code, you should know this means the powertrain control module (PCM) has detected a problem with the catalytic converter heater control circuit for engine bank 1. The first bank indicates the malfunction occurred in the catalytic converter for the engine bank containing cylinder #1. Conditions that can cause this code to be stored may be electrical or mechanical.

The main function of the heated catalytic converter is to reduce exhaust emissions from diesel and gasoline engines. It’s a dense filtration element made of ceramic fibers and precious metals enclosed in a steel housing and positioned in the exhaust pipe. Harmful nitrogen oxides (N2O), carbon monoxide, and unburned hydrocarbons are all converted into harmless nitrogen, oxygen, carbon dioxide, and water ions after passing through the heated catalytic converter. This is accomplished using the filtration element and the extreme heat from the engine exhaust.

In the case of the heated catalytic converter, the filtration element is further heated with a PCM-controlled injection of a certain type of flammable diesel exhaust fluid. Catalytic converter temperatures must reach at least 800 degrees Fahrenheit, and heated catalyst units can reach 1,200 degrees Fahrenheit.

The catalytic converter’s efficiency is monitored using upstream and downstream oxygen sensors (O2) and exhaust temperature sensors.

If the heated catalyst isn’t functioning properly, the oxygen concentration variations between input and output exhaust won’t be significant. If the upstream and downstream O2 sensors reflect similar exhaust oxygen concentration, or if an electrical malfunction is detected in the heated catalyst control circuit, a P0429 code will be stored and a malfunction indicator light may illuminate.

Other bank 1 catalyst efficiency trouble codes include:

P0420 Catalyst System Efficiency Below Threshold (Bank 1)
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0422 Main Catalyst Efficiency Below Threshold (Bank 1)
P0423 Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424 Heated Catalyst Temperature Below Threshold (Bank 1)
P0425, P0426, P0427, P0428 Catalyst Temperature Sensor Circuit Codes

Code Severity and Symptoms

Since the catalytic converter heater is crucial for exhaust emission reduction, a P0429 code should be considered severe.

Symptoms of this code may include:

Decreased fuel efficiency
Lack of overall engine performance
Excessive black smoke from exhaust
Other associated diagnostic codes
MIL (Malfunction Indicator Lamp) illumination

Causes

Possible causes for setting this code are as follows:

Faulty exhaust temperature sensor
Incorrect or insufficient diesel exhaust fluid
Faulty diesel exhaust fluid injection system
Burned, rubbed, broken, or disconnected wiring and/or connectors
Faulty O2 sensor(s)
Bad catalytic converter
Engine exhaust leak

Diagnostic and Repair Procedures

A diagnostic scanner, digital volt ohmmeter (DVOM), infrared thermometer (with laser pointer), and a reliable vehicle information source (like All Data DIY) will be needed when diagnosing a P0429 code.

All diesel exhaust fluid injection codes, misfire codes, throttle position sensor codes, manifold air pressure codes, and mass airflow sensor codes should be addressed before attempting to diagnose a stored P0429 code. The engine must be in good working condition before diagnosing this code.

I typically begin my diagnosis with a visual inspection of the system’s wiring harnesses and connectors. I would focus attention on harnesses routed near hot exhaust pipes and manifolds, as well as those routed near sharp edges like those found on exhaust shields.

Next, I would connect the scanner to the vehicle’s diagnostic port and retrieve all stored trouble codes and freeze frame data. I write down this information as it may prove helpful if this turns out to be an intermittent code. Clear the codes and test drive the vehicle to see if P0429 resets.

If applicable, ensure the diesel exhaust fluid tank is filled with the correct fluid and the injection system is functioning properly. If the diesel exhaust fluid injection system fails, the heated catalyst won’t function efficiently and a P0429 code will be stored. If the diesel exhaust fluid injection system isn’t working properly, check the system fuses and relays to ensure the controller is functioning.

If the diesel fluid injection system is working as expected, use the infrared thermometer to test the catalytic converter’s input and output temperatures. Use the vehicle information source and compare actual temperature data with manufacturer specifications. If the output temperature doesn’t meet specifications, suspect a faulty catalytic converter.

If the heated catalyst output temperature meets specifications, use the DVOM to test the exhaust temperature sensor using specifications found in the vehicle information source. Replace the exhaust temperature sensor if it doesn’t meet manufacturer specifications.

Test the O2 sensors following manufacturer recommendations.

Additional diagnostic notes:

P0429 code is frequently stored due to incorrect or insufficient diesel exhaust fluid
Disconnect associated controllers before probing circuits with the DVOM

P042A Catalyst Temperature Sensor Circuit Malfunction, Bank 1 Sensor 2

What does it mean?

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the oxygen sensor (O2) signals, the powertrain control module (PCM) can determine whether the catalytic converter is functioning properly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volt indicates a lean air/fuel mixture, while 0.9 volts indicates a rich mixture. If the converter is working correctly, the downstream sensor should read steadily around 0.45 volts.

The catalytic converter’s efficiency and temperature go hand in hand. If the converter is operating as it should, the outlet temperature should be slightly higher than the inlet. The old rule of thumb was a 100-degree Fahrenheit difference. However, many modern vehicles might not show that much of a difference.

There is no actual “catalyst temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “Bank 1” part of the code indicates the problem is with the engine’s first bank. That is, the bank that includes cylinder number 1. “Sensor 2” refers to the sensor mounted downstream of the catalytic converter.

Trouble code P042A is set when the PCM detects a malfunction in the Bank 1 Sensor 2 catalyst temperature sensor circuit.

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P042A engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Causes

Possible causes of this P042A code include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the downstream oxygen sensor and its associated wiring. Look for loose connections, damaged wiring, etc. Also, check for exhaust leaks visually and audibly. An exhaust leak can cause a false oxygen sensor code. If damage is found, repair as needed, clear the code, and see if it returns.

Next, check for Technical Service Bulletins (TSBs) regarding the issue. If nothing is found, you’ll need to move on to step-by-step diagnosis of the system. The following is a generalized procedure, as testing for this code varies by vehicle. To accurately test the system, you will need to refer to a diagnostic flowchart specific to the vehicle’s make/model.

Check for Other DTCs

Check Sensor Operation

To do this, it’s best to use a scan tool or, even better, an oscilloscope. Since most individuals don’t have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn the scan tool on and choose the Bank 1 Sensor 2 voltage parameter from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode.

The sensor should have a steady reading of 0.45 volts with only very small fluctuations. If it’s not responding correctly, it will likely need to be replaced.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before proceeding, you’ll want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles, and ALLDATADIY offers single-vehicle subscriptions. To check for continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor that will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Next, you’ll want to check the ground side of the circuit. To do this, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads Out of Limits (OL), there is an open circuit on the ground side of the circuit that will need to be located and repaired. If the meter reads a numerical value, there is continuity to ground.

Finally, you’ll want to verify that the PCM is processing the O2 sensor signal correctly. To do this, leave all connectors connected and insert a back probe pin into the PCM’s signal terminal. Set the digital multimeter to the DC volts setting. With the engine warmed up, compare the voltage reading on the meter to that on the scan tool. If the two do not match, the PCM is likely faulty or requires reprogramming.

P042B Catalyst Temperature Sensor Circuit Range B1S2

What Does It Mean?

The catalytic converter is one of the most important emission control devices on a vehicle. Exhaust gases pass through the catalytic converter where a chemical reaction occurs. This reaction converts carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into harmless water (H2O) and carbon dioxide (CO2).

The converter’s efficiency is monitored by two oxygen sensors; one mounted upstream of the converter and one mounted downstream. By comparing the signals from the oxygen sensor (O2), the powertrain control module (PCM) can determine if the catalytic converter is functioning correctly. A standard zirconia pre-catalyst O2 sensor will rapidly switch its output signal between approximately 0.1 and 0.9 volts. A reading of 0.1 volts indicates a lean air/fuel mixture, while 0.9 volts indicates a rich mixture. If the converter is working properly, the downstream sensor should read steadily around 0.45 volts.

There is no actual “catalyst temperature sensor.” What the codes described in this article refer to is the oxygen sensor. The “Bank 1” part of the code indicates the issue is with the engine’s first bank. That is, the bank that includes cylinder number 1. “Sensor 2” refers to the sensor mounted downstream of the catalytic converter.

Trouble code P042B is set when the PCM detects a problem with the range or performance in the Bank 1 Sensor 2 catalyst temperature sensor circuit.

Code Severity and Symptoms

The severity of this code is moderate. Symptoms of a P042B engine code may include:

Check Engine Light illuminated
Poor engine performance
Reduced fuel economy
Increased emissions

Possible Causes of this P042B Code Include:

Faulty oxygen sensor
Wiring issues
Unbalanced air/fuel mixture
PCM programming / faulty PCM

Diagnostic and Repair Procedures

Start by visually inspecting the downstream oxygen sensor and its corresponding wiring. Look for loose connections, damaged wiring, etc. Also check for exhaust leaks visually and audibly. An exhaust leak can cause a false oxygen sensor code. If damage is found, repair as necessary, clear the code, and see if it returns.

To do this, it’s best to use a scan tool or, even better, an oscilloscope. Since most individuals don’t have access to an oscilloscope, we will cover diagnosing the oxygen sensor using a scan tool. Connect the scan tool to the OBD port under the dashboard. Turn on the scan tool and choose the Bank 1 Sensor 2 voltage parameter from the data list. Bring the engine to operating temperature and view the sensor’s operation on the scan tool in graph mode.

The sensor should have a steady reading of 0.45 volts with only very small fluctuations. If it does not respond correctly, it will likely need to be replaced.

Check the Circuit

Oxygen sensors produce their own voltage signal which is sent back to the PCM. Before continuing, you will want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles and ALLDATADIY offers single-vehicle subscriptions. To check continuity between the sensor and the PCM, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the PCM’s O2 sensor signal terminal and the signal wire. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the sensor which will need to be located and repaired. If the meter reads a numerical value, there is continuity.

Next, you will want to check the ground side of the circuit. To do this, turn the ignition key to the “off” position and disconnect the O2 sensor connector. Connect a digital multimeter set to ohms (ignition off) between the O2 sensor connector ground terminal (harness side) and chassis ground. If the meter reads Out of Limits (OL), there is an open circuit on the ground side of the circuit which will need to be located and repaired. If the meter reads a numerical value, there is continuity to ground.