Error P0401 – how to fix?

Code Definition P0401

Insufficient exhaust gas recirculation flow detected

What does the code P0401 mean?

P0401 is a generic OBD-II code which indicates that the engine control module (ECM) has detected that the engine exhaust gas recirculation (EGR) valve is not flowing enough exhaust gas recirculation when commanded to open the gas flow to the intake manifold.

What causes the P0401 code?

• The EGR valve does not open fully due to the low vacuum opening the valve from the EGR control solenoid or the EGR is unable to maintain the vacuum.
• The EGR temperature sensor does not register a sufficient change in temperature as the EGR valve opens.
• The EGR lines are partially clogged and do not allow sufficient exhaust gas flow into the intake manifold.

What are the symptoms of the P0401 code?

• The Check Engine light will come on and the code will be stored in the ECM’s memory.
• The engine may beep or knock when accelerating.
• The engine fails to meet the emission requirements due to excess NOx in the exhaust tests.

How can a mechanic diagnose the P0401 code?

• Scan the codes and document the data from the frozen frame to verify the problem.
• Erase the engine codes and ETCs and test the vehicle on the road to see if the codes reappear.
• Visually inspect the vacuum hoses, lines and connections to the EGR valve and control coil, and the EGR temperature sensor.
• Disconnect and verify that the EGR valve receives sufficient vacuum from the engine when the control solenoid opens at light to moderate acceleration.
• Check that the EGR temperature sensor changes and the engine stalls when the EGR is opened.
• Remove the EGR valve and temperature sensor to check for excessive carbon build-up or partial blockage of the EGR lines from the valve to the intake manifold.

The most common errors in diagnosing the P0401 code

• Failure to check and clean the EGR lines when replacing a damaged EGR valve.
• Condemnation of a good EGR valve when the EGR temperature sensor has excessive carbon build-up on it causing an error code.
• Failure to verify that the EGR control solenoid maintains a vacuum prior to replacing the EGR valve.

How serious is the P0401 code?

• Failure of the EGR valve can cause excessive pre-ignition in the engine, which can cause internal damage to the piston and valves.
• Illumination of the Check Engine light will cause the vehicle to fail the NOx test.

What repairs can fix the code P0401?

1. Replace a leaking or clogged EGR valve.
2. Replace the damaged vacuum line to the EGR valve or control solenoid.
3. If the EGR temperature sensor is not registering a sufficient change in temperature, it must be replaced or carbon deposits removed to repair it.
4. Clean any carbon deposits from the EGR lines to the intake manifold to clear any obstructions.

Additional considerations about the P0401 code

Code P0401 triggers when the EGR temperature sensor does not see a sufficient change in temperature when the EGR is commanded to open. These sensors have a tendency to accumulate a large amount of carbon deposit, which causes them to lose their sensitivity to the heat of EGR gases.