P04001 min
P0400: EGR Flow Malfunction Causes & Fixes
P0400: EGR Flow Malfunction Causes & Fixes. See symptoms, common causes, driving risk and checks that help confirm the fault before parts are replaced.
Read diagnostic guide →Diagnostic system
Emissions, EVAP and Catalyst Trouble Codes
Work through EGR, secondary-air, catalytic-converter and EVAP faults while avoiding the common mistake of replacing expensive parts too early.
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Emissions codes need system-level diagnosis
A catalyst-efficiency code does not automatically condemn the converter, and an EVAP leak code does not identify the leak location. Exhaust leaks, combustion problems, sensor behavior and monitor conditions all matter.
Protect expensive repairs
Correct misfires, fuel-control problems, oil consumption and coolant entry before fitting a converter. For EVAP faults, inspect the cap and visible hoses before moving to smoke testing and valve-control checks.