What does the P0401 OBD-II diagnostic code mean?

Also known as insufficient EGR flow, this particular OBD-II trouble code indicates the engine control module (ECM) has detected the exhaust gas recirculation (EGR) valve is not allowing sufficient flow of recirculated exhaust gases into the engine. At this point, the P0401 error code is set. These recirculated exhaust gases are required in the engine’s combustion chambers to decrease the combustion temperature, reducing the formation of nitrogen oxides. 

How serious is the P0401 trouble code?

A P0401 diagnostic trouble code is considered a moderately serious issue. A defective EGR valve creates excessive ignition/pre-ignition that can cause internal engine damage to the valves and piston. If ignored, it will result in a breakdown, so it is advisable to repair the issue as soon as possible once the EGR issue has been determined. In addition, the ‘check engine’ light will cause the car to fail emission testing from the excess nitrogen oxide gases.

What are the common symptoms of a P0401 code?

If a P0401 OBD-II code is triggered, you can expect the following symptoms:

  • Check engine light illuminated 
  • Engine ignition ping or knock on acceleration
  • Slower acceleration
  • Failed emissions test from excess NOx

What are the causes of a P0401 error code?

There are many causes for the P0401 trouble code activating. The most common are:

  • Faulty or clogged EGR valve
  • Low vacuum opening the EGR valve from the EGR control solenoid resulting in the EGR valve not opening all the way
  • Carbon buildup in the intake manifold
  • Carbon buildup to the EGR temperature sensor
  • Defective EGR temperature sensor

How to diagnose a P0401 OBD-II code?

The easiest way to diagnose an OBD-II fault code is to use an OBD-II scanner or book a diagnostic check with a trusted mechanic or garage. 

  • Scan your system for other OBD-II codes.
  • Clear the fault codes, followed by a test drive of the vehicle while reading the live data on an OBD-II scanner to scan and document freeze-frame data to help verify the issue.
  • If the code persists after a road test, visually inspect vacuum hoses, wiring and connections to the EGR valve, control solenoid and the EGR temperature sensor.
  • Check that the EGR valve is getting enough engine vacuum to the valve when the control solenoid opens on light-to-medium acceleration.
  • Check the EGR valve and temperature sensor for any carbon buildup.
  • Inspect EGR tubes for partial blockages from the valve to the intake manifold.
  • Check intake manifold for carbon buildup

How to fix a P0401 OBD-II trouble code?

It’s advisable to test drive the car after each check/work is actioned upon to see if the fault code clears. If it returns, move on to the next repair. The most common repairs to resolve a P0401 trouble code are:

  • Confirm the code using an OBD-II scanner, and reset the fault code. Follow this up with a test drive of the vehicle to see if the trouble code clears. If it returns:
  • Replacing/repairing any broken or damaged vacuum hoses, wiring and connections to the EGR valve, control solenoid and/or EGR temperature sensor
  • Remove and clean any carbon buildup to the EGR Valve
  • Remove and clean any carbon buildup to the EGR temperature sensor
  • Removing carbon buildup/ partial blockages out of EGR tubes to the intake manifold 
  • Replace defective EGR temperature sensor
  • Replace defective EGR valve

How to avoid a P0401 diagnostic code?

Whilst it is highly possible that a faulty part such as the EGR valve is the root cause of a P0401 error code, carbon buildup is also a very common source. As such, the best way to avoid this particular diagnostic code is to maintain your vehicle’s engine in good, clean working order. Visually checking for problems with wiring, connectors and hoses and servicing your car regularly, including changing oil and filters, will ensure your vehicle remains in good running order.

About Fixter

Fixter is revolutionising the car maintenance industry, one repair at a time. Fixter was founded to make car maintenance as easy as booking a taxi. Digital, transparent and stress-free, with world-class customer service. Since launching in Manchester in 2017, Fixter has expanded to more than 100 towns and cities across the UK and provided thousands of car owners with honest, convenient and affordable car repair services.