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EmissionsSeverity Medium: diagnose soon

P0400Exhaust Gas Recirculation "A" Flow

What it means

The ECU sees EGR flow that doesn't match what it commanded, without being able to say whether there is too much or too little.

Symptoms

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

Common causes

  • EGR valve sticking (carbon)
  • Blocked EGR passages or cooler
  • Faulty EGR actuator or position sensor
  • Vacuum supply problem on vacuum-operated valves

What to check first

  1. Command the EGR open and closed and watch MAF and the valve position
  2. Remove and inspect the valve and passages
  3. Check the position sensor signal
Can I drive with P0400? 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 P0400

EGR valveWiring, typical values, multimeter and scope tests

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P0400: questions and answers

What does P0400 mean?

P0400: Exhaust Gas Recirculation "A" Flow. The ECU sees EGR flow that doesn't match what it commanded, without being able to say whether there is too much or too little.

What are the symptoms of P0400?

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

What causes P0400?

EGR valve sticking (carbon). Blocked EGR passages or cooler. Faulty EGR actuator or position sensor. Vacuum supply problem on vacuum-operated valves.

How do you diagnose P0400?

1. Command the EGR open and closed and watch MAF and the valve position 2. Remove and inspect the valve and passages 3. Check the position sensor signal

Can I drive with P0400?

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