P0401 Exhaust Gas Recirculation Flow Insufficient Detected

P0401 means the EGR monitor detected insufficient exhaust-gas-recirculation flow. The system commanded or expected EGR flow but did not see the expected engine response.

Code details

Generic OBD-II · P0

System: Emissions
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace the EGR valve before checking clogged passages and feedback sensors; low flow can come from restriction, control, or measurement faults.

Symptoms of P0401

Check-engine light.
Light knock or ping under load on some engines.
Rough idle or occasional stalling.
A failed emissions test.

Common causes of P0401

Carbon-clogged EGR passages or a sooted valve.
A failed EGR valve or its position sensor.
A blocked or leaking EGR tube/cooler where fitted.
A DPFE or EGR pressure-sensor fault.

Useful checks before replacing parts

Check EGR valve operation, passages, vacuum or electrical control, differential-pressure feedback, and carbon blockage.
Review freeze-frame data to see when the EGR monitor failed and whether other airflow or MAP codes are present.
Confirm flow with the manufacturer test procedure rather than judging by valve appearance alone.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.