What does P0401 mean?

Less exhaust gas flows through the EGR system than the ECU expects.

Can I clean the EGR valve to fix P0401?

Often yes, if carbon is the cause. Also clean the passages in the intake manifold.

Emission control

P0401 — Exhaust Gas Recirculation Flow Insufficient Detected

The ECU expects a certain amount of exhaust gas to flow back into the intake through the EGR system, but less flow is detected. NOx emissions rise and the engine may knock or idle roughly.

Severity: Medium

Common causes

Clogged EGR passages or carbon-fouled EGR valve
EGR valve stuck closed or faulty
Faulty EGR position/DPFE sensor
Vacuum line problem (on vacuum-operated valves)
Blocked EGR cooler

Symptoms

Pinging or knocking when accelerating
Rough idle
Failed emissions test
Check engine light

How to check

Inspect EGR valve and passages for carbon build-up
Check vacuum supply or electrical signal to the valve
Command the valve with a scan tool and watch idle response and sensor feedback
Clean or replace the valve, passages or sensor

Can you keep driving?

Usually drivable, but knocking under load can harm the engine over time. Repair it soon.

Frequently asked questions

What does P0401 mean?: Less exhaust gas flows through the EGR system than the ECU expects.
Can I clean the EGR valve to fix P0401?: Often yes, if carbon is the cause. Also clean the passages in the intake manifold.

General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.