EGR flow malfunction
What P0400 means
Your car's computer set P0400: EGR flow malfunction. It belongs to the emission controls group of codes. It is a generic code, so it means the same thing on every make. The check engine light usually comes on once the fault has been seen on two drives in a row. Until then it shows up as a pending code.
Can I keep driving? Usually safe to drive for now, but get it checked soon. Left alone it can lower fuel economy, fail an emissions test or lead to a bigger repair.
Common causes
- Carbon-clogged EGR valve or passages
- Failed EGR valve
- Cracked EGR vacuum hose
Fix the cheapest, most likely cause first. Clearing the code without a repair only turns the light off until the fault comes back.
Check it yourself
A rough idle together with an EGR code often means the valve is stuck open.
Torque Deck reads P0400 with a cheap Bluetooth OBD2 adapter. It also shows the freeze frame (engine conditions at the moment the code set) and the live data you need to narrow it down.
Parts
Parts most often replaced for P0400: EGR valve. Confirm the cause first, and match the part to your exact engine.
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Related codes
This page explains what the code usually means. It is not a diagnosis of your car. When in doubt, ask a mechanic.