P0400Powertrain
Exhaust gas recirculation (EGR) — malfunction
Where to start
The general exhaust gas recirculation code: the unit did not see the expected flow. As with P0401, in the vast majority of cases this is soot in the passage rather than a dead valve. Taking it off and looking takes ten minutes and costs nothing.
What to watch in live data
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
Step by stepHow to check the EGR valve with live data
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineEmission controls: catalyst, EGR, fuel vapour
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Exhaust gas recirculation (EGR)»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.