Technical Description of OBD-II Trouble Code
Correlation between Exhaust Gas Recirculation Temperature Sensor A/B
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to OBD-II equipped vehicles such as Ford, Seat, Chevy, GMC, Ford, Dodge, Sprinter, VW, Subaru, etc. Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
If your OBD-II equipped vehicle has stored code P040F, it means that the powertrain control module (PCM) has detected a discrepancy between the primary and secondary (A and B) exhaust gas recirculation (EGR) temperature sensors. This stored code may be the result of a mechanical issue or an electrical malfunction.
The EGR system’s purpose is to recycle a portion of spent exhaust gases back to the intake manifold for a second burn. This process is necessary to reduce the release of nitrous oxide (NOx) particles into the atmosphere. NOx emissions are regulated by federal mandate as they contribute to ozone depletion in exhaust emissions.
EGR temperature sensors are typically found in diesel engine equipped vehicles. In most cases, engine coolant is used to lower the temperature of engine exhaust gases before they enter the EGR valve. The A and B EGR temperature sensors provide the PCM with information about variations in exhaust gas temperature near the EGR valve. By comparing input signals from the two EGR temperature sensors and an exhaust temperature sensor, the PCM determines if the EGR cooling system is functioning efficiently.
The EGR temperature sensors are located near the EGR valve, while the exhaust temperature sensor is usually positioned in the down pipe. If the input voltage signals from the EGR temperature sensors do not meet programmed specifications, or if the input signals from the EGR temperature sensors are not significantly lower than that of the exhaust temperature sensor, a P040F code will be stored and the malfunction indicator lamp may illuminate.
Other trouble codes related to EGR temperature sensors include P040A, P040B, P040C, P040D, P041A, P041B, P041C, P041D, P041E, and P041F.
Code Severity & Symptoms
Since P040F is related to the exhaust gas recirculation system, it is not considered a severe code.
Symptoms of this code may include:
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
To diagnose a P040F, I would use a diagnostic scanner, a digital volt/ohmmeter, a reliable vehicle information source (All Data DIY), and an infrared thermometer (with a laser pointer).
You can start by visually inspecting the wiring harnesses and connectors associated with the A and B EGR temperature sensors and the exhaust temperature sensor. Pay close attention to harnesses that are near hot exhaust pipes and manifolds.
Next, connect the scanner to the vehicle diagnostic connector and retrieve all stored codes and freeze frame data. Write down this information as it may be needed if the code is intermittent. Clear the codes and test drive the vehicle to see if the code is reset.
If the P040F code is reset, use the scanner data stream to determine if there is actually a correlation problem between the two EGR temperature sensors or if there is a PCM or programming error. By narrowing down the scanner data stream to include only relevant information, you can obtain a quicker and more accurate data response. If the scanner indicates that the actual temperature inputs are within specifications, suspect a defective PCM or a PCM programming error.
If the EGR temperature sensor data is erratic or not within desired parameters, test the sensors according to the manufacturer’s recommendations. Replace sensors that fail to meet the manufacturer’s specifications. If the sensors are functioning properly, begin testing the EGR temperature sensor circuitry. Make sure to disconnect all related controllers before testing with the DVOM. Repair or replace any open or shorted circuits as necessary.
If the EGR temperature sensor electrical systems are functioning normally, use the infrared thermometer to check the exhaust gas temperature at the EGR cooler inlet and outlet while the engine is running at normal operating temperature. Compare your findings with the manufacturer’s specifications and replace any faulty EGR cooling system components as necessary.
Additional diagnostic notes:
