OBD-II · Engine and powertrain (P)
P0400
Exhaust gas recirculation (EGR) flow fault
What it means
The engine computer detects incorrect EGR flow. EGR feeds some exhaust gas back into the intake to lower NOx; diesels used in short trips are prone to soot build-up in the system.
Symptoms
- engine warning light
- possibly reduced power (limp mode)
- rough idle or hesitation
- more smoke than usual
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | carbon-clogged EGR valve |
| 02 | blocked EGR cooler or passages |
| 03 | EGR actuator or control solenoid fault |
| 04 | air-mass meter fault (diesels use it to judge EGR flow) |
| 05 | wiring fault |
What to check, step by step
- 01Read live data: EGR commanded vs actual position and air-mass readings.
- 02Run the EGR actuator test and check the valve moves freely.
- 03Remove and inspect the valve for carbon; clean or replace.
- 04Check the air-mass meter and intake for leaks.
- 05Inspect EGR cooler and intake manifold for blockage.