Error P0401 – how to fix it?

Code definition P0401

Insufficient exhaust gas recirculation flow detected

What does code P0401 mean?

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

What causes code P0401?

• The EGR valve does not open completely due to low vacuum opening, the EGR control solenoid valve or the EGR is not able to maintain vacuum.
• The EGR temperature sensor does not register enough temperature changes when the EGR valve opens.
• The EGR lines are partially clogged and do not allow sufficient exhaust gas flow to the intake manifold.

What are the symptoms of code P0401?

• The Check Engine light will come on, and the code will be stored in the ECM memory.
• The engine may beep or thump when accelerating.
• The engine does not meet the emission requirements due to excessive NOx in the exhaust tests.

How can a mechanic diagnose a P0401 code?

• Scan the codes and document the freeze frame data to verify the problem.
• Delete the engine and ETC codes and test the vehicle on the road to see if the codes reappear.
• Visually check the vacuum hoses, hoses and connections to the EGR valve and control coil, and the EGR temperature sensor.
• Turn off and check that the EGR valve receives sufficient vacuum from the engine when the control solenoid opens under light to moderate acceleration.
• Check if the EGR temperature sensor changes and if the engine dies when the EGR opens.
• Remove the EGR valve and temperature sensor to check for excessive carbon buildup or partial blockage of the EGR lines from the valve to the intake manifold.

The most common errors in diagnosing code P0401.

• Failed to check and clean the EGR lines when replacing a faulty EGR valve.
• Condemning a good EGR valve when the EGR temperature sensor has excessive carbon buildup, causing an error code.
• Failure to check that the EGR control solenoid maintains vacuum before replacing the EGR valve.

How serious is the code P0401?

• EGR valve failure can cause excessive pre-ignition in the engine, which can cause internal damage to the piston and valves.
• If the Check Engine light comes on, the vehicle will fail the NOx emissions test.

What repairs can fix a P0401 code?

1. Replacement of a leaking or clogged EGR valve.
2. Replace a faulty vacuum line through the EGR valve or control coil.
3. If the EGR temperature sensor does not register sufficient temperature changes, replace it or remove carbon buildup from it to repair it.
4. Clean the EGR lines to the carbon intake manifold to remove any blockages.

Additional notes to consider on code P0401.

The P0401 code is triggered when the EGR temperature sensor does not see enough temperature change when the EGR is commanded to open. These sensors tend to accumulate a lot of carbon buildup, causing them to lose sensitivity to the heat of the EGR gases.