Modern vehicles use an exhaust gas recirculation (EGR) system to reduce harmful emissions. When that system allows too much exhaust gas back into the engine, the powertrain control module (PCM) sets trouble code P0402.
Diagnostic trouble code (DTC) P0402 stands for “Exhaust Gas Recirculation ‘A’ Flow Excessive Detected.” It means the PCM has determined that too much exhaust gas is flowing through the EGR system when it shouldn’t be.
P0402 is a generic OBD-II code that can appear on many makes and models. However, the exact definition and diagnostic steps can vary by manufacturer, so the vehicle-specific service information should always be consulted.
The EGR system opens a pintle to allow about 5% of the exhaust gas to flow into the intake plenum where it displaces oxygen during cruise and helps reduce nitrogen oxide (NOx) emissions by lowering combustion temperatures inside the engine. When operating conditions are correct (throttle open, engine loaded), the PCM commands the EGR valve to open and allows a controlled amount of exhaust gas into the intake manifold.
These inert exhaust gases reduce peak combustion temperatures, which limits NOx formation. On gas burning engines, the EGR should never be open at idle. Some diesel engines will open the EGR at idle. It quietens the “diesel rattle” a bit. Because EGR is only needed under certain conditions, the PCM closely monitors how much flow is occurring on both gas and diesel engines equipped with EGR.
If excessive flow is detected, the PCM turns on the check engine light and stores code P0402.
Symptoms related to P0402 often affect drivability, though they can vary by vehicle. Common signs include:
Possible causes of P0402 include:
P0402 is not just an emissions-related code. Excessive EGR flow can cause rough idle, stalling, and poor engine performance.
Ignoring the problem can lead to worsening drivability and may contribute to further engine-related issues over time. For these reasons, the code should be diagnosed and repaired as soon as possible.
Because P0402 is a generic code, diagnostic procedures can vary by manufacturer and model year. A proper diagnosis usually follows a logical path:
Always follow manufacturer-specific diagnostic steps, especially for vehicles known to use DPFE sensors, such as many Ford models.
Fixing P0402 depends on the underlying cause identified during diagnosis. Common repair paths include:
Diagnosis should always come first to avoid unnecessary part replacement.
If you’re not confident in your diagnostic skills or automotive repair experience, it’s best to let a qualified mechanic handle the issue. Professional diagnosis can save time, prevent incorrect repairs, and ensure the underlying problem is properly resolved.
P0402 is triggered when the PCM detects that the EGR system is allowing too much exhaust gas flow under conditions where it should be limited.
No. While P0402 is a generic code, the exact definition and repair process can vary by manufacturer and vehicle model.
Yes. When the PCM detects excessive EGR flow, it illuminates the check engine light and stores the code.
Yes. A failing or sticking EGR valve is one of the possible causes of excessive EGR flow.
The code is often seen in vehicles that use a DPFE sensor to monitor EGR flow, including many Ford models
Yes. Since the EGR system directly controls NOx emissions, excessive EGR flow can disrupt proper emissions control.
Yes. While the vehicle may still run, the drivability issues and potential for further problems make prompt repair important.
Basic checks may be possible for experienced DIYers, but manufacturer-specific procedures and testing equipment are often required for accurate diagnosis.
Ignoring the code can lead to worsening drivability problems, such as stalling and rough idle, and may contribute to additional engine issues over time.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.