P048C Exhaust Pressure Control Valve Circuit Range

What Does It Mean?

This generic powertrain diagnostic trouble code (DTC) generally applies to all OBD-II equipped vehicles with an exhaust pressure control valve sensor or switch. This may include, but is not limited to, vehicles from VW, Audi, Toyota, etc. Although generic, the exact repair steps may vary by make/model.

The Exhaust Pressure Control (EPC) valve is a solenoid valve used to regulate backpressure during cold weather. This allows for increased cabin heat, facilitates cold starting, and windshield defrosting.

In most cases, the Powertrain Control Module (PCM) uses information from the Exhaust Back Pressure (EBP) sensor, Intake Air Temperature (IAT) sensor, and Manifold Absolute Pressure (MAP) sensor to determine valve control. If the PCM detects a problem with the EPC or IAT, it will disable the ECP. Typically, the ECP is found on diesel engines.

Code P048C is set when the PCM detects a performance issue in the exhaust pressure control valve circuit.

How Severe Is This DTC?

The severity of this code is moderate to severe. It’s a good idea to address this code as soon as possible.

What Are Some of the Symptoms of the Code?

Symptoms of a P048C trouble code may include:

Check Engine Light
Increased emissions
Poor engine performance
Hard starting

What Are Some Common Causes of the Code?

Causes

of this code may include:

Faulty exhaust pressure control valve
Wiring issues
Faulty PCM

What Are the P048C Troubleshooting Steps?

Start by checking the exhaust pressure control valve and its corresponding wiring. Look for loose connections, damaged wiring, etc. If damage is detected, repair as necessary, clear the code, and see if it returns. Next, check Technical Service Bulletins (TSBs) regarding the issue. If nothing is found, you’ll need to proceed with step-by-step system diagnostics.

The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, you’ll need to refer to the manufacturer’s diagnostic flowchart.
Check the Wiring

Before continuing, you’ll want to consult factory wiring diagrams to determine which wires are which. Autozone offers free online repair manuals for many vehicles, and ALLDATA offers single-vehicle subscriptions.
Test the Solenoid

Remove the solenoid connector. Use a digital multimeter set to ohms to check the solenoid’s internal resistance. To do this, connect the multimeter between the solenoid B+ terminal and the solenoid ground terminal. Compare the resistance measurement to factory repair specifications. If the meter displays a reading outside specifications or out of limits (OL) indicating an open circuit, the solenoid must be replaced.
Check the Power Side of the Circuit

Ensure the vehicle has been sitting for at least a few hours (overnight is preferable) and is cold. Remove the solenoid connector. With the ignition on, use a digital multimeter set to DC volts to check the solenoid’s power (usually 12 volts). To do this, connect the multimeter’s negative lead to ground and the multimeter’s positive lead to the solenoid B+ terminal on the harness side of the connector. If voltage is not present, set the meter to ohms (ignition off) between the solenoid connector B+ terminal and the solenoid power supply voltage terminal at the PCM. 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.

If everything is okay up to this point, you’ll want to check that the PCM is powered. To do this, turn the ignition on and set the meter to DC volts. Connect the meter’s positive lead to the EPC power supply voltage terminal at the PCM and the negative lead to ground. If there is no reference voltage from the PCM, the PCM is likely faulty. However, PCMs rarely fail, so it’s a good idea to recheck your work up to this point.
Check the Ground Side of the Circuit

With the ignition off, use a digital multimeter set to ohms to check for ground continuity. Remove the solenoid connector. Connect the multimeter between the solenoid ground terminal and chassis ground. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the solenoid that will need to be located and repaired.

P048D Low Value in Exhaust Pressure Control Valve Circuit

Low Circuit Value of Exhaust Pressure Control Valve Position Sensor/Switch

What Does It Mean?

This generic powertrain diagnostic trouble code (DTC) typically applies to all OBD-II equipped vehicles with an exhaust pressure control valve sensor or switch. This may include, but is not limited to, vehicles from VW, Audi, Toyota, etc. Although generic, the exact repair steps may vary by brand/model.

The Exhaust Pressure Control (EPC) valve is a solenoid valve used to regulate backpressure during cold weather. This helps increase cabin heat, facilitates cold starts, and aids in windshield defrosting.

In most cases, the Powertrain Control Module (PCM) uses information from the Exhaust Back Pressure (EBP) sensor, Intake Air Temperature (IAT) sensor, and Manifold Absolute Pressure (MAP) sensor to control the valve. If the PCM detects an issue with the EPC or IAT, it will disable the ECP. Typically, the ECP is found on diesel engines.

The P048D code is set when the PCM detects a low circuit signal from the exhaust pressure control valve. This usually indicates a short circuit in the circuit.
How Severe Is This DTC?

The severity of this code is moderate to severe. It is advisable to address this code as soon as possible.

What Are Some Symptoms of the Code?

Symptoms of a P048D trouble code may include:

Check Engine Light
Increased emissions
Poor engine performance
Hard starting

What Are Some Common Causes of the Code?

Causes

of this code may include:

Faulty exhaust pressure control valve
Wiring issues
Faulty PCM

What Are the Troubleshooting Steps for P048D?

Start by checking the exhaust pressure control valve and its associated wiring. Look for loose connections, damaged wiring, etc. If damage is found, repair as needed, clear the code, and see if it returns. Next, check for Technical Service Bulletins (TSBs) related to the issue. If nothing is found, proceed with step-by-step system diagnostics.

The following is a generalized procedure, as tests for this code vary by vehicle. To accurately test the system, refer to the manufacturer’s diagnostic flowchart.
Check the Wiring

Before proceeding, consult factory wiring diagrams to identify the wires. Autozone offers free online repair manuals for many vehicles, and ALLDATA provides single-vehicle subscriptions.
Test the Solenoid

Disconnect the solenoid connector. Use a digital multimeter set to ohms to check the solenoid’s internal resistance. Connect the multimeter between the solenoid’s B+ terminal and the solenoid’s ground terminal. Compare the resistance reading to factory repair specifications. If the meter shows a reading outside specifications or out of limits (OL) indicating an open circuit, replace the solenoid.
Check the Power Side of the Circuit

Ensure the vehicle has been sitting for at least a few hours (preferably overnight) and is cold. Disconnect the solenoid connector. With the ignition on, use a digital multimeter set to DC volts to check the solenoid’s power (typically 12 volts). Connect the multimeter’s negative lead to ground and the positive lead to the solenoid’s B+ terminal on the harness side of the connector. If no voltage is present, set the meter to ohms (ignition off) between the solenoid connector’s B+ terminal and the PCM’s solenoid supply voltage terminal. If the meter reads out of limits (OL), there is an open circuit between the PCM and the sensor that needs to be located and repaired. If the meter shows a numerical value, there is continuity.

If everything is fine up to this point, check that the PCM is powered. To do this, turn the ignition on and set the meter to DC volts. Connect the meter’s positive lead to the PCM’s EPC supply voltage terminal and the negative lead to ground. If there is no reference voltage from the PCM, the PCM is likely faulty. However, PCMs rarely fail, so it’s a good idea to double-check your work up to this point.
Check the Ground Side of the Circuit

With the ignition off, use a digital multimeter set to ohms to check ground continuity. Disconnect the solenoid connector. Connect the multimeter between the solenoid’s ground terminal and chassis ground. If the meter shows a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the solenoid that needs to be located and repaired.

P048E High Exhaust Pressure Control Valve Circuit

What does it mean?

The exhaust pressure control (EPC) valve is a solenoid valve used to regulate backpressure during cold weather. This allows for increased cabin heat, facilitates cold starting, and windshield defrosting.

In most cases, the powertrain control module (PCM) uses information from the exhaust backpressure (EBP) sensor, intake air temperature (IAT) sensor, and manifold absolute pressure (MAP) sensor to determine valve control. If the PCM detects a problem with the EPC or IAT, it will disable the ECP. Typically, the ECP is found on diesel engines.

Code P048E is set when the PCM detects a high exhaust pressure control valve circuit signal. This usually indicates an open circuit.

How severe is this DTC?

The severity of this code is moderate to severe. It’s a good idea to address this code as soon as possible.

What are some of the symptoms of the code?

Symptoms of a P048E trouble code may include:

Check engine light illuminated
Increased emissions
Poor engine performance
Hard starting

What are some common causes of the code?

Causes of this code may include:

Faulty exhaust pressure control valve
Wiring issues
Faulty PCM

What are the troubleshooting steps for P048E?

Start by checking the exhaust pressure control valve and its corresponding wiring. Look for loose connections, damaged wiring, etc. If damage is found, repair as necessary, clear the code, and see if it returns. Then, check for technical service bulletins (TSBs) related to the issue. If nothing is found, you’ll 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’ll need to refer to the manufacturer’s diagnostic flowchart.*

Check the wiring

Test the solenoid

Remove the connector from the solenoid. Use a digital multimeter set to ohms to check the internal resistance of the solenoid. To do this, connect the multimeter between the solenoid B+ terminal and the solenoid ground terminal. Compare the resistance measurement to factory repair specifications. If the meter displays a reading out of specifications or out of limits (OL) indicating an open circuit, the solenoid must be replaced.

Check the power side of the circuit

Ensure the vehicle has been sitting for at least a few hours (overnight is preferable) and is cold. Remove the connector from the solenoid. With the ignition on, use a digital multimeter set to DC volts to check the solenoid power (usually 12 volts). To do this, connect the multimeter’s negative lead to ground and the multimeter’s positive lead to the solenoid B+ terminal on the harness side of the connector. If voltage is not present, set the meter to ohms (ignition off) between the solenoid connector B+ terminal and the PCM’s solenoid supply voltage terminal. 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.

If everything is okay up to this point, you’ll want to check that the PCM is powered. To do this, turn the ignition on and set the meter to DC volts. Connect the meter’s positive lead to the PCM’s EPC supply voltage terminal and the negative lead to ground. If there is no reference voltage from the PCM, the PCM is likely faulty. However, PCMs rarely fail, so it’s a good idea to double-check your work up to this point.

Check the ground side of the circuit

With the ignition off, use a digital multimeter set to ohms to check for ground continuity. Remove the connector from the solenoid. Connect the multimeter between the solenoid ground terminal and chassis ground. If the meter reads a numerical value, there is continuity. If the meter reads out of limits (OL), there is an open circuit between the PCM and the solenoid that will need to be located and repaired.

P048F Intermittent Exhaust Pressure Control Valve Circuit

Intermittent Exhaust Pressure Control Valve Position Sensor/Switch Circuit

What does it mean?

The Exhaust Pressure Control (EPC) valve is a solenoid valve used to regulate backpressure during cold weather. This helps increase cabin heat, aids cold starting, and windshield defrosting.

Code P048F is set when the PCM detects an intermittent issue with the exhaust pressure control valve circuit.
How severe is this DTC?

The severity of this code is moderate to severe. It’s a good idea to address this code as soon as possible.

What are some of the symptoms of the code?

Symptoms of a P048F fault code may include:

Check Engine Light
Increased emissions
Poor engine performance
Hard starting

What are some common causes of the code?

Causes

of this code may include:

Faulty exhaust pressure control valve
Wiring issues
Faulty PCM

What are the troubleshooting steps for P048F?

Remove the solenoid connector. Use a digital multimeter set to ohms to check the solenoid’s internal resistance. Do this by connecting the multimeter between the solenoid B+ terminal and the solenoid ground terminal. Compare the resistance measurement to factory repair specifications. If the meter shows a reading outside specifications or Out of Limits (OL) indicating an open circuit, the solenoid must be replaced.
Check the power side of the circuit

Ensure the vehicle has sat for at least a few hours (overnight is preferable) and is cold. Remove the solenoid connector. Ignition on, use a digital multimeter set to DC volts to check for solenoid power (typically 12 volts). Do this by connecting the multimeter’s negative lead to ground and the positive lead to the solenoid B+ terminal on the harness side of the connector. If voltage is not present, set the meter to ohms (ignition off) between the solenoid connector B+ terminal and the PCM’s solenoid supply voltage terminal. 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.

If everything is okay up to this point, you’ll want to verify that the PCM is powered. To do this, turn the ignition on and set the meter to DC volts. Connect the meter’s positive lead to the PCM’s EPC supply voltage terminal and the negative lead to ground. If there is no reference voltage from the PCM, the PCM is likely faulty. However, PCMs rarely fail, so it’s a good idea to double-check your work up to this point.
Check the ground side of the circuit

Ignition off, use a digital multimeter set to ohms to check for ground continuity. Remove the solenoid connector. Connect the multimeter between the solenoid ground terminal and chassis ground. If the meter reads a numerical value, there is continuity. If the meter reads Out of Limits (OL), there is an open circuit between the PCM and the solenoid that will need to be located and repaired.

P0490 High Exhaust Gas Recirculation (EGR) Control Circuit “A

Understanding Error Code P0490: EGR Control Circuit “A” – High Value

Introduction

What does the P0490 code mean? (General definition: high voltage value detected in the EGR valve “A” control circuit).
This is a generic powertrain code (OBD-II), applicable to all makes and models from 1996 onwards.
Important: Specific troubleshooting steps vary depending on the vehicle.
These engine fault codes indicate an electrical malfunction of the EGR (Exhaust Gas Recirculation) system.
Essential role of EGR: reduce the formation of harmful nitrogen oxides (Nox) in the cylinders.
EGR operation: controlled by the engine management computer (PCM/ECU) which opens or closes the valve based on load, speed, and temperature to maintain an optimal cylinder head temperature.
Key components:
- Electric solenoid (usually two wires) for valve activation.
- Potentiometer integrated into the solenoid that signals the computer the position of the EGR pintle (opening/closing of the passage).
Analogy with a light dimmer: the computer attempts to modulate the voltage to control the EGR opening/closing.
The P0490 code indicates that the computer does not detect a corresponding high voltage change when attempting to open or close the EGR, suggesting the valve is potentially stuck in a position.
Distinction from code P0489: Fundamentally identical, but indicates a low circuit instead of a high circuit.
Importance of EGR for:
- Reducing Nox emissions.
- More advanced ignition timing for more power without knocking.
- A leaner air-fuel mixture for better fuel economy.

Possible Symptoms of Code P0490

Symptoms vary depending on the position of the EGR pintle at the time of failure:
- Engine running extremely rough
- Check Engine light illuminated
- Decreased fuel economy
- Loss of power
- No start or very hard start followed by rough idle

Potential Causes of Code P0490

List of possible causes:
- Short to ground
- Short to battery voltage
- Bad connector with pushed-out pins
- Corrosion in the connector
- Clogged EGR pintle
- Faulty EGR solenoid
- Defective EGR valve
- Faulty ECU or computer

Repair Procedures for Code P0490

Step 1: Warranty and TSB Check:
- If the vehicle has less than 160,000 km (100,000 miles), check the warranty (often 130,000 to 160,000 km for emission controls).
- Search online for relevant Technical Service Bulletins (TSB) for this code and your vehicle.
Tools needed for diagnosis:
- Voltmeter / Ohmmeter (multimeter)
- EGR system wiring diagram specific to your vehicle
- Jumper wire
- Two paperclips or thin sewing needles
Step 2: EGR Mechanical Blockage Test (Stall Test):
- Open the hood and start the engine.
- If the engine is idling, disconnect the EGR valve connector.
- If the engine idle improves, the EGR pintle is likely stuck in the open position.
- Stop the engine and replace the EGR valve.
Step 3: Visual Inspection of Connector and Wiring:
- Carefully examine the EGR valve connector (usually 5 wires).
- Identify the power (battery voltage) and ground wires (usually the two outer wires).
- The three center wires are related to the EGR position potentiometer (including a 5-volt reference terminal).
- Look for pushed-out pins, corrosion, or bent pins in the connector.
- Inspect the wiring harness for any damaged insulation or potential short circuit.
- Look for broken wires that could cause an open circuit.
Step 4: Testing EGR Valve Power Supply:
- With the voltmeter, probe one of the end terminals of the EGR connector (red wire) and place the black wire to the vehicle ground.
- Turn the ignition on (without starting the engine) and check for the presence of 12 volts on both end terminals (power and ground).
- If no voltage is displayed, there is a break in the power wire between the EGR and the ignition circuit.
- If 12 volts are present on only one side (the power), the EGR valve may have an internal open circuit. Replace the EGR.
Step 5: Testing EGR Valve Activation with a Jumper:
- Remove the EGR valve connector.
- Turn the ignition on (engine off). Probe the two outer terminals to identify the power supply (12 volts).
- Place a paperclip in the end terminal that did not have power (this is the ground terminal).
- Attach the jumper wire to the paperclip and place the other end of the jumper to the vehicle ground.
- You should hear an audible “click” when the EGR valve is activated.
- Disconnect the jumper from ground and start the engine.
- Reconnect the jumper to ground. The engine idle should become rough when the EGR is powered and stabilize when the ground is removed.
- If the EGR clicked and affected the engine idle, the EGR valve is likely functional and the problem is electrical. Otherwise, stop the engine and replace the EGR.
Step 6: Testing the EGR Potentiometer Reference Circuit:
- Probe the center terminal of the EGR connector with the voltmeter (red wire). Place the black wire to the vehicle ground.
- Turn the ignition on (engine off). You should measure approximately 5.0 volts if the computer is working correctly. Turn the ignition off.
Step 7: Testing the Reference Circuit at the ECU/PCM:
- Use the EGR wiring diagram to locate the “EGR Reference Voltage” terminal on the computer (ECU/PCM) connector.
- Insert a fine pin or paperclip into the computer connector at that terminal to probe it.
- Turn the ignition on. If 5 volts are present, the computer is likely good and the problem is in the wiring harness between the computer and the EGR.
- If no voltage is present, the computer could be faulty.
Alternative Solution (Workaround) for a Faulty Reference Circuit at the ECU/PCM:
- Consult the wiring diagram and locate the reference voltage terminal for the engine coolant temperature sensor.
- Probe this terminal with the ignition on. If a 5-volt reference voltage is present, turn the ignition off and mark the two reference terminals used (EGR and coolant temperature).
- Disconnect the computer connector and solder a jumper wire between these two reference terminals.
- Reinstall the connector. The EGR should function normally without replacing the computer. Note: This is a temporary repair and it is recommended to diagnose and repair the root cause of the ECU problem if possible.

P0491 Insufficient Flow in Secondary Air Injection System, Bank 1

Insufficient Flow of Secondary Air Injection System (Bank 1)

What Does It Mean?

This is a generic powertrain code, meaning it covers all makes/models from 1996 onward.

However, specific troubleshooting steps vary by vehicle.

The secondary air injection system, commonly found on Audi, BMW, Porsche, and VW vehicles—and may be found on others—includes an air pump, exhaust manifold, intake check valve, vacuum control switch, electrical input circuit for the vacuum control switch, and numerous vacuum hoses.

This system pumps fresh air into the exhaust system during cold starts to supply oxygen and exhaust gases. This ensures more complete combustion of harmful emissions—hydrocarbons. After about a minute, the system shuts off.

If the air pump and check valve are faulty, they are usually easier to replace; however, most vacuum lines are located under or behind the intake manifold, making them hard to access in most cases. Code P0491 indicates a problem with the system—typically that the secondary air flow is too low on bank 1. Bank #1 is the engine side with cylinder #1. For bank #2, see code P0492.

Other secondary air injection system fault codes include P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0418, P0419, P041F, P044F, and P0492.

Symptoms

Symptoms of a P0491 engine code may include:

Malfunction Indicator Lamp (MIL) illumination

Potential Causes

Potential causes of a P0491 may include:

Faulty exhaust manifold check valve
A fuse or relay for the secondary air injection pump may be faulty
A faulty air pump
Leaking intake hose
A faulty vacuum control switch
A disconnected vacuum line
A leak in the hoses/piping between the secondary air injection pump and the combi or secondary air injection
A secondary air pressure sensor may be faulty
The combi valve itself is faulty
The secondary air injection port in the cylinder head may be clogged with carbon
The secondary air injection ports in the cylinder head may be obstructed

Diagnostic and Repair Procedures

Most diagnostic tests should be performed when the engine is completely cold. Some follow-up tests may need to wait until the engine cools down completely again—usually a few hours. Some tests may also generate additional error codes. Remember to clear all error codes after finding and fixing the issue.

1. Check the pump. Ensure the engine is completely cold. Remove the pressure hose from the pump or the manifold check valve—whichever is easier to access. Have someone start the engine while you check that the pump is running and pumping air from the end of the hose or output fitting (depending on where you disconnected it). If air is being pumped, proceed to step 4; otherwise, go to step 2.

2. Disconnect the pump’s electrical harness connector. Apply 12 volts to the pump using jumper wires. The terminal with the brown wire is ground, and the other terminal is positive. If the pump runs, proceed to step 3; otherwise, the pump must be replaced.

3. Ensure the engine is cold, connect the multimeter, then have someone start the engine for this step. Test the pump harness connector to ensure it has 12 volts by checking the power between the two terminals of the pump harness plug. If you have 12 volts, repeat the first three steps to ensure no mistakes were made. If you don’t have 12 volts, check the fuses and relay. If you find a bad fuse or relay, replace it, then repeat this step to ensure the harness is powered.

4. Ensure the engine is completely cold. Remove the pressure hose from the check valve. Check if air is coming out of the hose when starting the engine. Do not turn off the engine. After a minute, does the valve close and stop the airflow? If so, the check valve is working correctly. If the valve did not operate, proceed to step 5. If the valve remains open, remove the vacuum control hose and start the engine—the engine does not need to be cold for this test. If the valve remains open, the valve is bad; if it closes, the vacuum control switch is likely the issue.

5. For this step, you’ll need a vacuum pump available at auto parts stores. Start the engine with the vacuum pump attached and holding a check valve nipple. If the valve is open, release the vacuum. If the valve closes, the valve is correct. If the valve does not open at all or does not close, then the valve must be replaced.

6. Let the engine cool completely. Attach the vacuum gauge to the control hose at the check valve. Start the engine. You should have at least 10 to 15 inches of vacuum. Otherwise, you may need to take the vehicle to a shop, as further diagnostic tests require removing some engine components. If no vacuum is present or vacuum readings are low, proceed to step 7.

7. Locate your vehicle’s vacuum control switch. Trace the vacuum line from the check valve to the switch. Look for hardened spots in the hose, cracks, or loose connections. Replace the hose if found, then repeat the test in step 6. If you still have no vacuum, you may need to remove engine parts to access the switch.

8. Remove the input vacuum hose from the control switch. Attach the vacuum gauge to the input hose to test manifold vacuum—with the engine running. If you have no vacuum, replace the vacuum hose and repeat this step.

9. Apply vacuum to the input nipple of the vacuum control switch. The valve should be closed, and the pump should not hold vacuum. Apply 12 volts to both terminals of the control switch using jumper wires. If the switch does not open and release vacuum from the pump, then the switch must be replaced.
Secondary Air Injection Video

P0493 Fan Overspeed

Fan Overspeed

What does it mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Chevrolet, Buick, Peugeot, Toyota, Citroën, Dodge, BMW, Honda, Acura, Ford, Dodge, etc.). Although generic, specific repair steps may vary by make/model.

The fan manufacturers refer to in these codes is the cooling fan. Generally, the fan is mounted on the radiator. Its function is to cool the coolant in the radiator and help regulate engine temperature.

The Engine Control Module (ECM) or Powertrain Control Module (PCM) monitors/controls a multitude of sensors to monitor and regulate engine temperature. The ECM activates the fan when engine temperatures become high (e.g., idling in traffic) and turns it off when airflow is sufficient to maintain low temperatures (i.e., driving on the highway).

While monitoring the cooling fan, the ECM detected an overspeed condition. If the cooling fan spins beyond or below the parameters set by the manufacturer, the check engine light or Malfunction Indicator Lamp (MIL) will be illuminated by the ECM.

Diagnostic codes related to the engine cooling fan:

P0494 Fan Speed Low
P0495 Fan Speed High

Symptoms

Symptoms of a P0493 engine code may include:

Engine overheating
Engine noises
Poor performance
Engine misfire
Surging
Poor fuel consumption

Potential Causes

Typically, the causes for this code to set are:

Broken or damaged wiring harness
Faulty fan clutch
Faulty cooling fan clutch or bearing
Faulty cooling fan relay
ECM problem
Pin/connector issue (e.g., corrosion)

Diagnostic and Repair Procedures

Whenever working with electrical systems, some basic tools that are a good idea are:

OBD code reader
Multimeter
Basic socket set
Ratchet and wrench sets
Basic screwdriver set
Rags/shop towels
Battery terminal cleaner
Service manual

Safety Tips

Let the engine cool down
Wear PPE (Personal Protective Equipment)

Basic Step #1

Visual Inspection. First, you’ll want to open the hood and locate your radiator/fan assembly. Most of the time, it will be mounted towards the front of the vehicle, directly in front of the engine. Keep in mind there are many different styles and configurations for cooling fans; some are mounted directly on the radiator itself, and some are belt-driven by the engine, so identify the style of your particular cooling fan. Once located, inspect the fan(s) and blades for any damage. It’s a good idea to grab one of the fan blades and shake it gently to check for any play in the bearings; depending on the style and the amount of play present, you may need to replace the fan. Also, be sure to inspect the shroud and anything that could hinder fan performance. Pay close attention to the cooling fan wiring harness. It only takes one slightly frayed wire to trigger certain codes. If everything is okay, continue.

TIP: Sometimes aftermarket shrouds don’t fit perfectly, causing the fan to rub on parts of the shroud, resulting in low speed. Spin the fan by hand with the engine off and cold. The fan should spin freely within the shroud (it may be necessary to remove the belt for the belt-driven version).
Basic Step #2

Check the Connector. First, ensure the battery is disconnected before performing any repairs or tests on the electrical system. Follow the wiring harness coming from the cooling fan until you find its connector. Once located, inspect for overheating/melting or corrosion. Disconnect it and inspect the pins inside. Any corroded or bent pins should be repaired/replaced. Generally, any damage to the connector means you will need to replace it. If everything is okay, continue.

TIP: Ensure you have a healthy electrical system. Always be aware that if you have a weak battery or an issue with your charging system, the ECM may falsely illuminate the check engine light among many other possibilities.
Basic Tip #3

Check the Relay. Depending on your system, you may have a cooling fan relay that helps control your fan. Refer to the manufacturer for the location. If the relay appears melted or burnt, this could indicate a problem with the relay and it should be replaced.

TIP: It’s a good idea to purchase these types of relays from your dealer, as I have found faulty aftermarket relays that caused serious damage.
Basic Step #4

Check your ECM or PCM. Locate your ECM. The location is highly dependent on the make and model, but you can find an ECM: under a seat, under the hood, in the trunk, mounted on the firewall, under the dashboard, in a fender, or floor-mounted somewhere. Once located, inspect for water damage or corrosion; this could indicate a problem. Skilled expertise would be needed beyond this point.

P0494 Low Fan Speed

Low Fan Speed

What Does It Mean?

While monitoring the cooling fan, the ECM detected a low-speed condition. If the cooling fan spins beyond or below the parameters set by the manufacturer, the check engine light or Malfunction Indicator Lamp (MIL) will be illuminated by the ECM.

Diagnostic Codes Related to Engine Cooling Fan:

P0493 Fan Over-speed
P0495 Fan High Speed

Symptoms

Symptoms of a P0494 engine code may include:

    Engine overheating
    Engine noises
    Poor performance
    Engine misfire
    Stalling
    Poor fuel consumption

Potential Causes

Typically, the causes for this code to set are:

    Broken or damaged wiring harness
    Faulty fan clutch
    Faulty cooling fan clutch or bearing
    Faulty cooling fan relay
    ECM problem
    Pin/connector issue (e.g., corrosion)

Diagnostic and Repair Procedures

Whenever working with electrical systems, some basic tools that are a good idea are:

    OBD code reader
    Multimeter
    Basic socket set
    Ratchet and wrench sets
    Basic screwdriver set
    Rags/shop towels
    Battery terminal cleaner
    Service manual

Safety Tips

    Let the engine cool down
    Chock the wheels
    Wear PPE (Personal Protective Equipment)

Basic Step #1

TIP: Sometimes aftermarket shrouds don’t fit perfectly, causing the fan to rub on parts of the shroud, leading to low speed. Spin the fan by hand with the engine off and cold. The fan should spin freely within the shroud (it may be necessary to remove the belt for the belt-driven version).
Basic Step #2

Check the Connector. First, ensure the battery is disconnected before performing any repairs or tests on the electrical system. Follow the wiring harness coming from the cooling fan until you find its connector. Once located, inspect for overheating/melting or corrosion. Disconnect it and inspect the pins inside. Any corroded or bent pins should be repaired/replaced. Generally, any damage to the connector means you’ll need to replace it. If everything is okay, proceed.

TIP: Ensure you have a healthy electrical system. Always be aware that if you have a weak battery or an issue with your charging system, the ECM may falsely illuminate the check engine light among many other possibilities.
Basic Step #3

TIP: It’s a good idea to purchase these types of relays from your dealer, as I have found faulty aftermarket relays that caused serious damage.
Basic Step #4

Check your ECM or PCM. Locate your ECM. The location heavily depends on the make and model, but you might find an ECM: under a seat, under the hood, in the trunk, mounted on the firewall, under the dashboard, in a fender, or floor-mounted somewhere. Once located, inspect for water damage or corrosion; this could indicate a problem. Skilled expertise would be needed beyond this point.

P0495 High Fan Speed

High Fan Speed

What does it mean?

This diagnostic code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II (Chevrolet, Buick, Peugeot, Toyota, Citroën, Dodge, BMW, Honda, Acura, Ford, Dodge, etc.). Although generic, specific repair steps may vary by brand/model.

The fan that manufacturers refer to in these codes is the cooling fan. Generally, the fan is mounted on the radiator. Its function is to cool the coolant in the radiator and help regulate the engine temperature.

While monitoring the cooling fan, the ECM detected a high-speed condition. If the cooling fan spins beyond or below the parameters set by the manufacturer, the engine light or Malfunction Indicator Lamp (MIL) will be illuminated by the ECM.

Diagnostic codes related to the engine cooling fan:

P0493 Fan Over-speed
P0494 Fan Low Speed

Symptoms

Symptoms of an engine code P0495 may include:

Engine overheating
Engine noises
Poor performance
Engine misfire
Surging
Poor fuel consumption

Potential Causes

Typically, the causes for this code to be set are as follows:

Broken or damaged wiring harness
Faulty fan clutch
Faulty cooling fan clutch or bearing
Faulty cooling fan relay
ECM problem
Pin/connector issue (e.g., corrosion)

Diagnostic and Repair Procedures

Whenever working with electrical systems, some basic tools that are a good idea to have are:

OBD code reader
Multimeter
Basic socket set
Ratchet and wrench sets
Basic screwdriver set
Rags/shop towels
Battery terminal cleaner
Service manual

Safety Tips

Let the engine cool down
Chock the wheels
Wear PPE (Personal Protective Equipment)

Basic Step #1

Check the Connector. First, ensure the battery is disconnected before performing any repairs or tests on the electrical system. Follow the wiring harness coming from the cooling fan until you find its connector. Once located, inspect for overheating/melting or corrosion. Disconnect it and inspect the pins inside. Any corroded or bent pins should be repaired/replaced. Generally, any damage to the connector means you’ll need to replace it. If everything is okay, proceed.

TIP: Ensure you have a healthy electrical system. Always be aware that if you have a weak battery or an issue with your charging system, the ECM may falsely illuminate the engine light among many other possibilities.
Basic Tip #3

Check the Relay. Depending on your system, you may have a cooling fan relay that helps control your fan. Refer to the manufacturer for the location. If the relay appears melted or burnt, this may indicate a problem with the relay and it should be replaced.

TIP: It’s a good idea to purchase these types of relays from your dealer as I have found faulty replacement relays that caused severe damage.
Basic Step #4

Check your ECM or PCM. Locate your ECM. The location heavily depends on the make and model, but you can find an ECM: under a seat, under the hood, in the trunk, mounted on the firewall, under the dashboard, in a fender, or floor-mounted somewhere. Once located, inspect for water damage or corrosion; this may indicate a problem. Skilled expertise would be needed beyond this point.

P0496 EVAP Flow During Non-Purge Condition

What Does It Mean?

This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II. Although generic, specific repair steps may vary by make/model.

A quick online search shows this code appears more common with GM vehicles (Chevy, Pontiac, etc.). In Acura, Honda, Hyundai, Kia, and Mazda vehicles, this code is described as “EVAP system high purge flow,” which is the same thing.

This DTC checks for unwanted intake vacuum flow to the EVAP system. The control module seals the EVAP system by commanding the EVAP canister purge solenoid valve OFF and the EVAP canister vent solenoid valve ON. The control module monitors the fuel tank pressure (FTP) sensor to determine if a vacuum is being developed in the EVAP system.

If the vacuum in the EVAP system exceeds a predetermined value within a set time, this code is set and the MIL (malfunction indicator lamp) illuminates.

Symptoms

Symptoms of a DTC P0496 include the MIL illuminating, and most likely no other noticeable symptoms. Some may experience hard starting. In some cases, the engine may run rich, which may not be noticeable but can cause long-term damage (e.g., catalytic converter damage).
Potential Causes

Potential causes of an EVAP P0496 code include:

Faulty purge or vent solenoid valve
Clogged EVAP canister
Faulty EVAP/fuel tank pressure sensor
Poor electrical connection
Electrical short condition
(Hyundai, Isuzu)
Faulty canister purge valve
Blocked vapor canister
Faulty vent solenoid
Leaking EVAP system hose (Hyundai)
Faulty purge flow sensor (Kia, Mazda)

Possible Solutions

The most common solution for this DTC is to replace the purge solenoid valve. However, ensure proper diagnosis before replacing parts!

Ideally, use an advanced scan tool with the ignition on and engine off, seal the EVAP system using the Seal/Purge function. Then, observe the fuel tank pressure sensor reading when disabling the purge. If the pressure value is outside the manufacturer’s specified range, replace the EVAP canister purge solenoid valve. Refer to a model-specific repair guide for proper specifications.

If you don’t have access to a scan tool, you can disconnect the vacuum line at the purge valve toward the charcoal canister. Disconnect the electrical connector on the purge valve, start the engine, and place your finger over the valve where the line was disconnected. If you feel vacuum, the purge solenoid valve is faulty and needs replacement. Alternatively, you can remove the purge solenoid valve and blow through it. It is normally closed, so if air passes through, it should be replaced.

If the purge valve checks out fine, the issue with the valve is intermittent, or there’s a problem with the fuel tank pressure sensor. To test the fuel tank pressure sensor, you’ll need a high-end scan tool to monitor tank pressure with the gas cap removed. If the sensor shows vacuum with the gas cap off, there’s an issue with the fuel tank pressure sensor.