What does P0400 mean?
The control module detected a condition described as “Exhaust gas recirculation flow malfunction”. The code identifies the affected area, but it does not by itself prove which component has failed. The code describes the detected condition, not a guaranteed failed part. A reliable diagnosis also considers related codes, freeze-frame data, live values and the actual symptoms.
Possible causes
- Carbon build-up in the EGR valve or cooler
- Vacuum supply or actuator fault
- Position sensor or wiring fault
- Restricted EGR passages or throttle assembly
Common symptoms
- Power loss and limp mode
- Hesitation at part load
- Smoke from the exhaust
- Higher fuel consumption or poor emissions
Sensible next checks
- Compare commanded and actual EGR position in live data
- Observe mass-air-flow changes while EGR is actuated
- Inspect connectors, vacuum lines and the mechanism
- Clean components only after the cause and correct access are confirmed
Why does it return after clearing?
Clearing the code only removes the stored entry. If the underlying condition remains, the control module will set the code again after the next relevant monitor cycle. Save the data first, diagnose the cause, repair it and only then clear the code.