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P0401: Exhaust gas recirculation (EGR) flow insufficient

Less gas is flowing through the EGR than the ECU asks for. It is nearly always carbon blocked passages or a valve that fails to open.

Code P0401 at a glance
CodeP0401
SystemEngine and transmission
GroupP04xx, Emissions control
TypeGeneric: it means the same on every make
Can you keep driving?With care

What P0401 means

The ECU opens the EGR valve and expects a change in the intake (pressure, temperature or air flow). If the change is smaller than expected, it concludes that too little exhaust gas is getting through. This is one of the most common EGR codes, especially on cars used mainly in town.

Most common causes

  • EGR passages blocked with carbon
  • Dirty EGR valve that does not open fully
  • Cracked or loose vacuum hose (vacuum operated valves)
  • Clogged EGR cooler
  • Faulty vacuum solenoid or EGR valve
  • Intake pressure or EGR position sensor giving wrong readings

Symptoms

  • Engine warning light on
  • Pinking (metallic knocking) under load on petrol engines
  • Failing an emissions test for NOx
  • Often no noticeable symptoms

Can you drive with P0401?

With care The car usually drives fine, but combustion runs hotter and petrol engines may start to knock. Get it checked soon, especially before an inspection.

How to check it

If your reader shows EGR position, check that it moves when the ECU commands it open. The fault is often in the passages rather than the valve itself, so ask the garage to check that gas can actually get through.

How to read and clear P0401 with Final OBD

Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0401 with this same explanation and whether it’s confirmed, pending or permanent.

With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.

Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.

Similar codes

P0401 is a generic code, so it means the same on any make, for example on Citroën, Hyundai, Mini, Renault, Seat, Volvo. See what Final OBD does on your car in makes and models.

What does code P0401 mean?

The ECU opens the EGR valve and expects a change in the intake (pressure, temperature or air flow). If the change is smaller than expected, it concludes that too little exhaust gas is getting through. This is one of the most common EGR codes, especially on cars used mainly in town.

Can you drive with P0401?

The car usually drives fine, but combustion runs hotter and petrol engines may start to knock. Get it checked soon, especially before an inspection.

How do you clear P0401?

With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0401 comes back.