What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Ford, Seat, Chevy, GMC, Ford, Dodge, Sprinter, VW, Subaru, etc.). Although generic, specific repair steps may vary by make/model.
If your OBD-II equipped vehicle has stored a P040F code, it means the powertrain control module (PCM) has detected a discrepancy between the primary and secondary exhaust gas recirculation (EGR) temperature sensors (A and B). A stored P040F code can result from a mechanical issue or an electrical malfunction.
The EGR system is designed to return some of the spent exhaust gases to the intake manifold so they can be burned a second time. This process is necessary to reduce nitrogen oxide (NOx) particles released into the atmosphere. NOx is a component contributing to ozone-depleting exhaust emissions, and NOx emissions are regulated by federal mandate.
EGR temperature sensors are (to my knowledge) relegated to diesel engine vehicles. In most cases, engine coolant is used to lower the temperature of the engine exhaust gases before they are drawn into the EGR valve. The EGR temperature sensors A and B provide the PCM with variations in exhaust gas temperature near the EGR valve. The PCM compares the input signals from both EGR temperature sensors and an exhaust temperature sensor to determine if the EGR cooling system is functioning effectively.
EGR temperature sensors are located near the EGR valve, and the exhaust temperature sensor is typically positioned in the downpipe. If the input voltage signals from the EGR temperature sensor do not meet programmed specifications, or if the input signals from the EGR temperature sensor(s) are not significantly lower than those from the exhaust temperature sensor, a P040F will be stored, and a malfunction indicator lamp may illuminate.
Other EGR temperature sensor fault codes include P040A, P040B, P040C, P040D, P041A, P041B, P041C, P041D, P041E, and P041F.
Code Severity and Symptoms
Since P040F is related to the exhaust gas recirculation system, it is not classified as a severe code.
Symptoms of this code may include:
- There may be no symptoms when this code is stored
- Reduced fuel efficiency
- Coolant leak
- Exhaust leak
- Exhaust temperature sensor codes
Causes
Possible causes for setting this code are as follows:
- Low coolant
- Faulty EGR temperature sensor(s)
- Faulty exhaust temperature sensor
- Exhaust leaks
- Clogged EGR cooler
- Engine overheating
Diagnostic and Repair Procedures
A diagnostic scanner, digital volt/ohmmeter, reliable vehicle information source (All Data DIY), and an infrared thermometer (with laser pointer) are some tools I would use to diagnose a P040F.
You can start with a visual inspection of the wiring harnesses and connectors associated with the EGR temperature sensors A and B and the exhaust temperature sensor. Carefully examine harnesses near hot exhaust pipes and manifolds.
Next, connect the scanner to the vehicle’s diagnostic connector and retrieve all stored codes and freeze frame data. Note this information as you might need it if it turns out to be an intermittent code. Now, clear the codes and test-drive the vehicle to see if it resets.
If P040F resets, I would use the scanner’s data stream to determine if there is an actual correlation problem between the two EGR temperature sensors or if there is a PCM or programming error. By narrowing the scanner’s data stream to include only relevant information, you can get a faster and more accurate data response. If the scanner indicates that actual temperature inputs meet specifications, suspect a faulty PCM or PCM programming error.
If EGR temperature sensor data is erratic or does not match desired parameters, test the sensors following manufacturer recommendations. Replace sensors that do not meet manufacturer specifications. If sensors function correctly, begin testing EGR temperature sensor circuits. Ensure all associated controllers are disconnected before testing with the DVOM. Repair or replace open or shorted circuits as needed.
If EGR temperature sensor electrical systems are functioning normally, use the infrared thermometer to check exhaust gas temperature at the EGR cooler inlet and EGR cooler outlet (with the engine running at normal operating temperature). Compare your results with manufacturer specifications and replace faulty EGR cooling system components if necessary.
Additional Diagnostic Notes:
- Aftermarket mufflers and other exhaust components can cause exhaust temperature variations that might trigger this code
- Exhaust backpressure issues, created by an inadequate diesel particulate filter (DPF), are known to contribute to P040F code storage conditions
- Diagnose and repair DPF-related codes before attempting to diagnose this code
- If the EGR system has been modified using an EGR blocking kit (currently offered by OEMs and aftermarket companies), this type of code may be stored