OBD2 codes.
A code identifies a signal or system that needs to be checked, not a part you should replace.
Got an OBD2 Bluetooth adapter? Read the codes straight from your car.1 code entry · 2 related faults
P0401The EGR valve is letting through less gas than expectedUrgency medium
Exhaust Gas Recirculation Flow Insufficient Detected
The EGR valve sends part of the exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxides (NOx). This code means that when the engine computer opens the EGR, less gas flows than expected.
What the car has detected
With EGR switched on, the engine computer has measured recirculated gas flow below the calibrated value (it works this out, for example, from how much the air measured by the MAF sensor drops).
You can usually drive, but the engine emits more NOx, may fail the inspection and, if the cause is carbon build-up, the problem tends to get worse. Get it checked soon.
Symptoms
- Engine warning light on
- Often no change when driving
- Knocking or pinking when accelerating on some petrol engines
- Failing the emissions test
Possible causes
- EGR valve or passages blocked with carbon
- EGR valve stuck closed
- Faulty EGR vacuum line or control valve
- On some diesels, a clogged EGR filter
- Air leaks between the turbo and the intake manifold
- Faulty mass air flow sensor
Checks for the garage
- Check there are no other codes and that the air filter isn't clogged
- Look for leaks in the intake and vacuum lines
- Check the mass air flow sensor, which can also set the code
- Move the EGR valve with diagnostics (from almost closed to almost open) and see whether it responds and the air flow changes
- Check whether the valve and passages are blocked with carbon
Next step
Have the EGR valve and its passages checked for carbon blockage, and the intake leaks and MAF sensor checked.
Go to a garage if the code comes back: cleaning or replacing the EGR usually means removing parts and using diagnostics to check it works.
Original sources · Date indicated: 2026-10-12
Related faults in the database
Cracked particulate filter and clogged EGRExhaust and emissions
- Engine / equipment
- 2.0 TDI common rail EA189 with low-pressure EGR, mainly US-market versions (CJAA, CBEA)
- Period mentioned
- Approx. 2009–2015
- Symptoms
- Engine light, limp mode with power loss, flashing glow-plug light and black soot inside the tailpipe.
- Possible cause described
- The diesel particulate filter (DPF) cracks internally and lets soot through, clogging the low-pressure EGR filter behind it.
- Check for the garage
- Remove the EGR filter and wipe inside the exhaust at that point: if the cloth comes out black, the DPF is cracked. Replace the DPF and EGR filter together.
- Cost cited in the source
- Approx. €1,000–2,500 (estimate: DPF + EGR filter)A rough figure from the original database, not an up-to-date quote.
Sources from the original database
EGR valve sticking with deposits on the 1.5 dCiExhaust and emissions
- Engine / equipment
- 1.5 dCi (K9K) with an electric EGR valve
- Period mentioned
- No specific period published
- Symptoms
- Uneven idle, jerks when accelerating, lack of power or the engine going into limp mode, sometimes with the engine warning light on.
- Possible cause described
- Soot and oil deposits leave the EGR valve stiff, stuck closed or not closing fully. According to Pierburg it is rarely a fault of the valve itself: the oil comes through the crankcase breather, a turbo with worn bearings or a blocked oil return, late oil changes or too high a level, and many short trips, above all in winter.
- Check for the garage
- At the workshop: read the codes with diagnostic equipment (EOBD P0400 to P0404 or P0172; Renault DF077, DF084 or DF241), test the valve electrically and its opening with the engine warm, and find where the oil comes from (breather, turbo, level). Pierburg advises against cleaning it, as it can be damaged: it is replaced if faulty, and an engine ECU update sometimes solves it.
- Cost cited in the source
- No verified estimate; ask the workshop for a quote for the EGR valve.A rough figure from the original database, not an up-to-date quote.