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P0401 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what does code p0401 means, how to fix it, and what other codes may show related to it.


P401 Exhaust Gas Recirculation Flow Insufficient Detected


When your vehicle pops up a P0401 trouble code on an OBD-II scanner, it means the  engine control module (ECM) detected a problem with the engine exhaust gas recirculation (EGR) valve. Generally this error is caused by insufficient recirculated exhaust gases when the ECM tells the EGR to allow exhaust gases to flow into the intake manifold.


The specific cause of your vehicle coding a P401 can have any number of explanations, but these are the most typical three we see when a vehicle’s ECM posts that trouble code:

  • The EGR valve cannot open properly due to a low vacuum pressure in the valve from the EGR control solenoid or the EGR not being able to hold vacuum.
  • The EGR temperature sensor doesn’t detect a sufficient temperature change when the EGR valve opens.
  • The EGR passages are partly blocked and aren’t providing sufficient exhaust gases flow to the engine intake manifold.


Just like the cause, symptoms can be different from vehicle to vehicle. The three most common are as follows:

  • The Check Engine Light will be illuminated and the P401 code is set in ECM memory.
  • The engine could have an engine ignition ping, hard idle, or knock on acceleration.
  • The engine will fail an emissions test due to excessive NOx detected in exhaust gases.


Your check engine light is a good indicator that you need to take your ride into a shop to get checked out. Make sure your shop has qualified technicians who know how to diagnose and troubleshoot ECM OBD-II scanner codes. Here’s our process for diagnosing and troubleshooting a P401 trouble code.

  • Our tech scans codes and documents the freeze-frame data to verify the problem and code.
  • Next, we clear the engine and ETC codes in the ECM, and then take your vehicle for a short road test to see if the problem reoccurs..
  • If codes are still coming up P401, our technician visually checks the vacuum hoses, wiring, and connections to the EGR valve and control solenoid, along with the EGR temperature sensor. Sometimes physical damage from wear and tear is evident upon visual inspection.
  • After the visual check, our tech will disconnect and check if the EGR valve is getting enough engine vacuum to the valve when the control solenoid opens on light-to-medium acceleration.
  • Our technician monitors the OBDII scanner for EGR temperature sensor changes, and to see if the engine starts to idle hard or run rough when the EGR is opened.
  • Finally, we remove the EGR valve and temperature sensor to check for excessive carbon buildup. Our technicians also inspect the EGR tubes from the valve to the intake manifold for partial blockage and excessive carbon buildup. Provided there is no damage to the EGR valve, temperature sensor or solenoid control, a thorough cleaning should fix the problem causing your P401 error code.

Common mistakes

Checking and repairing the cause of a P401 code has a few pitfalls that are common to the replacement process. It’s easy to have a vehicle code P401 again if you or your technician makes any of the following common mistakes:

  • Failing to check and clean the EGR passages when replacing a defective EGR valve (any excess carbon  build up that isn’t scrubbed out can come loose and jam or damage the EGR valve).
  • Replacing your EGR valve when the EGR temperature sensor is causing the P401 code issue because it has excessive carbon buildup.
  • Failing to check to see if the EGR control solenoid will hold vacuum before replacing the EGR valve.

EGR valves are pricey replacement parts, and you want to make certain that the valve is faulty before going through the expensive and time-consuming process of scrubbing out the EGR system, cleaning the sensors, hoses, and valve tubes on the intake manifold. You or your technician also needs to ensure that every part of the EGR system is spotless so as not to damage any replacement parts you’ve just installed.

How Serious

A check engine light is always serious, but is a P401 error a reason to rush your car into the shop right away? We’d say so. Here’s why:

  • A failed EGR valve creates excessive ignition/pre-ignition, causing internal engine damage to the piston and valves. Eventually, this is a broke-down-by-the-roadside type of issue, so you want to fix the problem ASAP after you have determined you have an EGR issue.
  • The Check Engine Light  causes the vehicle to fail emission testing from excessive NOx gases. In states where emissions inspections are mandatory to register your vehicle, this must be corrected before you can legally operate the vehicle.

What repairs can fix the code

  • Replacing the leaking or clogged EGR valve (the worst case scenario)
  • Replacing a broken vacuum line to the EGR valve or control solenoid (better scenario)
  • 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 (not a terrible option)
  • Cleaning carbon out of EGR tubes to intake manifold to clear any blockages (time-consuming but worthwhile since it prevents a much more expensive repair).

*One caveat worth noting: if your owner’s manual states that your engine needs 91 octane gasoline or better, don’t run your engine on anything less than 91 octane gasoline. Running low-octane gas on a high-octane engine is one of the primary causes of excess carbon buildup in your vehicle’s EGR system. Paying more for gas might sting a bit, but it is far less expensive in the long run than paying for a replacement EGR valve.

P0401 Code – What Does It Mean & How To Fix It