P0400 code meaning
EGR A Flow
Powertrain system Generic code P04xx codes Check engine light
What P0400 means
Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx. P0400 is the general flow fault of EGR system A, used where the module does not classify the error as too little (P0401) or too much (P0402). Ford gasoline engines with a stepper-motor valve open and close the valve at steady part load and compare intake manifold pressure with and without EGR; too small a normalised difference (below 1.0) sets P0400, MIL after two trips. GM diesels use the code differently, for air-mass checks with excessive EGR rate. EGR is then usually switched off.
P0400 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor1 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P0400
- Check engine light on
- Pinging under load or rough idle, depending on direction
- Higher NOx, emissions test failed
Common causes of P0400
- Most likelyEGR passages or valve carboned up
- PossibleEGR valve stiff or faulty
- PossibleLeak in the intake or EGR pipe
- Less likelyMAP or MAF sensor inaccurate
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
EGR valveEGR passagesMAP sensorEngine control module
What each part of P0400 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 00 | The exact fault in that area. |
What to do next
- Note P0400 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0400
What does P0400 mean?
P0400 means "EGR A Flow". Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx. It is a generic code, so it means the same on every make.
Is it safe to drive with P0400?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0400 fail an emissions test?
Yes, it can. P0400 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0400 to fix?
Difficulty: medium. A typical repair takes 1 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0400 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.