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 |
|---|---|
| System | Engine and transmission |
| Group | P04xx, Emissions control |
| Type | Generic: 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.
Open the bonnetthe in-depth explanation
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 its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
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 an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P0400 Exhaust gas recirculation (EGR) flow malfunction
- P0402 Exhaust gas recirculation (EGR) flow excessive
- P0403 EGR control circuit malfunction
- P0404 EGR circuit range or performance problem
- P0405 EGR sensor A circuit low
- P0406 EGR sensor A circuit high
- P0407 EGR sensor B circuit low
- P0408 EGR sensor B circuit high
- P0409 EGR sensor A circuit malfunction
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: EGR and particulate filter.
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.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
