P0401: Insufficient exhaust-gas recirculation flow

What P0401 means

Verified description: Exhaust Gas Recirculation Flow Insufficient Detected

General OBD-II EGR explanation for vehicles that support P0401. Gasoline and diesel procedures differ. This guide does not establish UTH Scanner support for any engine or module.

Vehicle-specific limits

• Gasoline and diesel EGR designs can differ substantially, as can vacuum-operated and electrically controlled systems.
• Some vehicles infer flow through pressure or other sensors. The code reports the monitor’s conclusion, not a direct visual measurement of a blocked passage.

What the code does and does not tell you

What it establishes

• The EGR flow monitor recorded less flow than expected for its test.

What it does not establish

• That the EGR valve is the only possible fault.
• That removing or disabling emissions equipment is an appropriate repair.

Symptoms you may notice

• A check-engine warning may appear.
• Report roughness, hesitation or a change in power if present; the complete complaint helps determine urgency.

Possible causes

• Restricted passages, a valve that does not open sufficiently or a control problem can limit flow.
• On applicable systems, vacuum supply, hoses or flow-feedback information may affect the result.

A possibility is not a diagnosis. Confirm the cause with the vehicle-specific testing procedure before replacing parts.

Sensible next checks

1. Save other engine or emissions codes and note any recent intake, exhaust or EGR work.
2. Limit an owner inspection to obvious loose accessible connectors or hoses with the engine off and cool. EGR components can be very hot.
3. Have a technician compare the commanded operation with the system’s response, then check passages, control and feedback using the correct service procedure.

When to get professional help

Professional inspection is sensible before cleaning or replacing EGR parts. The confirmed restriction, control fault or feedback problem should explain the result and be checked again after repair.

Related codes

• P0300 — Random/Multiple Cylinder Misfire Detected
• P0171 — System Too Lean (Bank 1)
• P0174 — System Too Lean (Bank 2)