Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC P0401: General Description
The exhaust gas recirculation (EGR) valve, which is controlled by the powertrain control module (PCM), is opened and the exhaust gas flows through the EGR valve, the intake manifold, and the EGR passage. The exhaust gas is circulated into the air/fuel mixture to lower combustion temperatures, thus reducing oxides of nitrogen (NOx) emissions. The EGR flow is inspected as follows. The EGR valve is closed during deceleration with the throttle valve fully closed. Then the PCM fully opens the EGR valve. After a set time, the PCM computes the ratio of the present EGR flow to the normal EGR flow by monitoring the fluctuation of the intake manifold pressure between when the EGR valve is fully opened and when it is fully closed. If the EGR flow rate is lower than normal, the PCM detects a malfunction and stores a DTC.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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