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.