P0401 Trouble Code Help | YourMechanic Advice

What is the P0401 Code?

The P0401 code is an OBD-II diagnostic trouble code (DTC) that the engine control module (ECM) detects when the engine exhaust gas recirculation (EGR) valve is not flowing enough recirculated exhaust gasses when it is commanded to open the flow of gasses into the intake manifold.

P0401 Code Causes & Symptoms

P0401 Code Causes

• The EGR valve is not opening all the way due to a low vacuum opening the valve from the EGR control solenoid or the EGR not being able to hold vacuum.
• The EGR temperature sensor is not registering enough of a temperature change when the EGR valve is opened.
• The EGR passages are partly blocked and will not allow enough flow of exhaust gasses into the intake manifold.

P0401 Code Symptoms

• The Check Engine Light will be illuminated and the code is set in ECM memory.
• The engine could have an engine ignition ping or knock on acceleration.
• The engine will fail emissions from excessive NOx in the exhaust tests.

P0401 Code Mechanic Diagnosis & Mistakes

P0401 Code Mechanic Diagnosis

• Scans codes and documents the freeze frame data in order to verify the problem.
• Clears the engine and ETC codes and road tests the vehicle to verify whether the codes come back.
• Visually checks the vacuum hoses, wiring, and connections to the EGR valve and control solenoid along with the EGR temperature sensor.
• Disconnects and checks the EGR valve is getting enough engine vacuum to the valve when the control solenoid opens on light-to-medium acceleration.
• Checks for EGR temperature sensor changes and engine to bog down when the EGR is opened.
• Removes EGR valve and temperature sensor to check for excessive carbon buildup or partial blockage of the EGR tubes from the valve to the intake manifold.

P0401 Code Common Mistakes

• Failing to check and clean the EGR passages when replacing a defective EGR valve.
• Condemning a good EGR valve when the EGR temperature sensor has excessive carbon buildup on it causing the failure code.
• Failing to check to see if the EGR control solenoid will hold vacuum before replacing the EGR valve.

P0401 Code Impact & Steps to Fix

How Serious is the P0401 Code?

• A failed EGR valve can cause the engine to have excessive pre-ignition that can cause internal engine damage to the piston and valves.
• The Check Engine Light on will cause the vehicle to fail emission testing from excessive NOx gasses.

How to Fix the P0401 Code

• Replacing the leaking or clogged EGR valve
• Replacing a broken vacuum line to the EGR valve or control solenoid
• Replacing an EGR temperature sensor or clean the carbon off of it to repair it if it does not register enough of a temperature change
• Cleaning carbon out of EGR tubes to intake manifold to clear any blockages

P0401 Code Additional Comments and Help

Code P0401 is triggered when the EGR temperature sensor does not see enough of a temperature change when the EGR is commanded to open. These sensors tend to get a lot of carbon buildup that causes them to lose sensitivity to the heat from the EGR gasses. YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.