Emissions · Fault code

P0401

Exhaust Gas Recirculation Flow Insufficient

The ECU commanded EGR flow and did not see enough of it. This is about flow, not the electrical circuit, and the usual culprit is carbon, not electronics.

Severity EXPERT Low — worth checking, not urgent
Driveability EXPERT Usually safe to drive while you diagnose
Tuning EXPERT Diagnose first — do not tune onto this

Check first — in this order

The order is the product. A list of "possible causes" is what every parts site publishes, and it is why people replace parts blindly.

  1. Command the valve and watch whether flow or the feedback temperature responds. A valve that moves but moves no gas points at blockage, not the valve. EXPERT
  2. Check the EGR passages and cooler for carbon blockage — the leading cause and invisible until it is opened up. EXPERT
  3. Check the valve for sticking and its actuator for travel. EXPERT
  4. Then the differential-pressure or temperature sensor the ECU uses to judge flow. EXPERT

What this code does not license you to replace

  • That the EGR valve has failed. A blocked passage with a healthy valve logs the same insufficient-flow code.
  • That cleaning the valve alone clears a blocked cooler downstream of it.

Companion codes

Codes whose presence alongside P0401 changes the path.

P0101
No workshop sign-off yet. The diagnostic order on this page has had a technical review of the knowledge model, performed by a model — not a manufacturer certification and not a mechanical inspection of any vehicle. No tuner has yet put their name to this ordering. Treat it as a reasoned starting point, confirm with measurement, and get a professional to sign off before you condemn a part.

Built by DMS Tuned

Stuck on P0401? Send us the log.

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