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P0400: Exhaust Gas Recirculation Flow
Severity: Low Usually safe to keep driving, but get it checked.
What does P0400 mean?
There is a general fault in the EGR system, which routes some exhaust back into the intake to lower combustion temperatures.
Generic OBD-II definition: Exhaust Gas Recirculation Flow. Confirm applicability in your vehicle service information.
Common causes
- Carbon-clogged EGR passages
- Stuck or faulty EGR valve
- Faulty EGR position sensor or DPFE sensor
- Cracked vacuum lines (older vehicles)
Symptoms
- Pinging/knocking under load
- Rough idle
Can I keep driving with P0400?
Severity: Low Usually safe to keep driving, but get it checked.
If the check engine light is flashing or the car runs badly, stop driving and have it checked.
Typical fixes
- Inspect EGR valve and passages
- Test valve operation