P0400: Exhaust Gas Recirculation Flow Malfunction

What it means

A general fault has been detected in the exhaust gas recirculation (EGR) system. EGR routes a portion of exhaust back into the intake to reduce combustion temperature and NOx emissions. Most EGR codes today are caused by carbon buildup blocking the valve.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The exhaust gas recirculation system routes a controlled amount of exhaust back into the intake to limit combustion temperature and emissions. P0400 sets when the module detects that EGR flow does not behave as expected, which can result from restricted passages, a valve that does not move correctly, or invalid flow or position feedback.

Common causes

Most likely first.

1. EGR valve clogged with carbon
2. Intake EGR passages blocked
3. DPFE / EGR position sensor failed

Symptoms

• Check engine light on
• Rough idle or hesitation
• Reduced engine performance
• Increased emissions

How to diagnose it

1. Save freeze-frame data and compare EGR command, position feedback, and any available flow-related live data to see when expected flow is missing or abnormal.
2. Inspect the EGR valve, connecting passages, and intake openings for carbon deposits or blockage.
3. Check the EGR valve connector and wiring, then inspect the position or DPFE sensor hoses and connections if fitted.
4. Use a scan tool's available actuator test to command EGR movement and see whether position feedback or engine response follows.
5. If deposits and wiring are not the cause, test the EGR position or flow sensor against the manufacturer's procedure.

Common fixes

• Clean carbon from the EGR valve and passages if inspection confirms restriction.
• Repair damaged EGR wiring, connectors, or sensor hoses.
• Replace a failed EGR position or DPFE sensor after testing confirms its readings are invalid.
• Replace an EGR valve that fails its movement or sealing checks.

Can I keep driving?

A mild hesitation may allow a short, gentle trip, but avoid heavy acceleration if the engine runs roughly or lacks power. The fault can increase emissions and, if EGR flow is incorrect, contribute to poor running.