EngineLightCodes
Powertrain code

P0400 EGR A Flow

Severity
Fix soon
Typical repair cost
$90–$660
Do it yourself
Possible, medium
Shop time
1–3 hours

Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake to lower combustion temperature and NOx.

How the engine computer detects it

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 normalized 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.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

Symptoms

  • Check engine light on
  • Pinging under load or rough idle, depending on direction
  • Higher NOx, emission test failed

Common causes

  1. EGR passages or valve carboned up Most likely
  2. EGR valve stiff or faulty Possible
  3. Leak in the intake or EGR pipe Possible
  4. MAP or MAF sensor inaccurate Less likely

How to diagnose P0400

  1. Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
  2. Inspect the parts involved: egr valve, egr passages, map sensor, engine control module.
  3. Work through the causes above from most to least likely, cheapest checks first.
  4. Clear the code and drive a full drive cycle to confirm the repair held.

Reading the code

P
Powertrain
0
Generic (SAE) code
4
Auxiliary emission controls
00
Specific fault

Questions drivers ask

Can I drive with code P0400?

Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.

Will my car pass an emissions inspection with P0400?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0400 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0400 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.