Powertrain, generic · Auxiliary emission controls

P0400 EGR A Flow

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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 petrol 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.

Likely causes

EGR passages or valve carboned up
EGR valve stiff or faulty
Leak in the intake or EGR pipe
MAP or MAF sensor inaccurate

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€80–600
Time1–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Inspect EGR valve and passages for carbon
  2. Command the valve with a scan tool
  3. Check vacuum lines

Parts involved

egr valveegr passagesmap sensorengine control module

Related codes

P0401P0402P0403P0404P0106

References: newtonnet.co.uk, fordservicecontent.com, gsi.ext.gm.com, law.cornell.edu.