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EmissionsSeverity Medium: diagnose soon

P0401Exhaust Gas Recirculation "A" Flow Insufficient Detected

What it means

When the ECU opens the EGR valve, the expected change (drop in MAF or rise in MAP) is too small: not enough exhaust gas is flowing back.

Symptoms

  • Rough idle or stalling
  • Loss of power, sometimes limp mode
  • Black smoke (diesel) or pinging under load (petrol)

Common causes

  • EGR valve or passages blocked with carbon
  • EGR cooler blocked
  • Faulty EGR actuator
  • Vacuum supply problem

What to check first

  1. Command the EGR open and watch the MAF drop
  2. Inspect the EGR valve and the intake passages
  3. Check the actuator position feedback
Can I drive with P0401? Usually yes for a short distance, but performance or economy may suffer and the fault can get worse. Diagnose it soon.

Generic OBD-II meaning. Manufacturer-specific meanings and checks can differ; always check the service information for the car.

How to test the parts behind P0401

EGR valveWiring, typical values, multimeter and scope tests

Known engine problems that set P0401

2.0 TDI EA189: EGR valve and EGR coolerVolkswagen, Audi, Skoda, Seat · typically 120,000–220,000 km1.6 TDI EA189: EGR valve stickingVolkswagen, Audi, Skoda, Seat · typically 100,000–200,000 kmBMW N47: EGR cooler leaking coolant (recall)BMW, Mini, Toyota · typically 60,000–200,000 km1.3 Multijet/CDTI: EGR valve cloggingFiat, Opel, Suzuki, Ford, Lancia, Alfa Romeo, Peugeot, Citroën · typically 80,000–180,000 kmToyota 2.0/2.2 D-4D: EGR and intake carbon build-upToyota, Lexus · typically 80,000–180,000 km1.8 Hybrid 2ZR-FXE: Clogged EGR system, misfire and head gasket damageToyota, Lexus · typically 150,000–300,000 km

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P0401: questions and answers

What does P0401 mean?

P0401: Exhaust Gas Recirculation "A" Flow Insufficient Detected. When the ECU opens the EGR valve, the expected change (drop in MAF or rise in MAP) is too small: not enough exhaust gas is flowing back.

What are the symptoms of P0401?

Rough idle or stalling. Loss of power, sometimes limp mode. Black smoke (diesel) or pinging under load (petrol).

What causes P0401?

EGR valve or passages blocked with carbon. EGR cooler blocked. Faulty EGR actuator. Vacuum supply problem.

How do you diagnose P0401?

1. Command the EGR open and watch the MAF drop 2. Inspect the EGR valve and the intake passages 3. Check the actuator position feedback

Can I drive with P0401?

Usually yes for a short distance, but performance or economy may suffer and the fault can get worse. Diagnose it soon.