Fault code P0401 – a step-by-step guide to diagnosing and fixing it

DTC P0401 is registered by the engine control unit (engine control module) when it determines that an insufficient amount of exhaust gas is being redirected through the car EGR valve. This recirculation process is supposed to lower the combustion temperatures and reduce harmful emissions. It is regulated based on the operating temperature and engine load.

• Valve faults caused by a broken cable, a weak spring, or a damaged rubber membrane, preventing the device from regulating the flow of gases correctly.
• An EGR valve that won’t open due to a low vacuum pressure. This may occur as a result of a leak in the vacuum lines, a solenoid malfunction, or a low pump output caused by a faulty drive belt.
• Blockages obstructing the flow of exhaust gases into the air intake manifold. This often occurs when carbon deposits form in the passages of the system.
• A jammed throttle valve caused by a build-up of carbon.
• A broken pressure feedback (DPFE) or temperature sensor, generating false signals or failing to transmit accurate data.

• An increase in nitrogen oxide emissions (a failed emissions test)
• Knocking or ignition pinging when accelerating
• Increased fuel consumption
• A rough idle
• Activation of the check engine light
• Surging or stalling

• One of the first things that a qualified technician would do is check the registered trouble codes. You can do this yourself by connecting an OBD2 scanner to the vehicle’s onboard computer via the data link connector. They would then clear the codes and take the vehicle for a test drive to see if they reappear.
• If there are vacuum leaks, restore the suction by replacing damaged vacuum lines, installing a new pump or tightening the clamps.
• Clean the valves to remove deposits and contaminants. This can be done by using cleaning additives or by hand with a clean, lint-free cloth, a brush and special cleaning agent, such as a throttle body cleaner.
• Inspect the wires and sensors for signs of damage or failure. Remove the temperature sensor and check whether carbon deposits have built up around the component causing problems. Check the voltage output of the DPFE device and compare the voltmeter or multimeter readings to the standard voltage range. Replace any defective electrical parts.
• You can conduct a visual test for the EGR valve using a hand-held vacuum pump or using a vacuum gauge. If the component won’t open or stay open when the vacuum pump is working, you should install a new one.