P0400
EGR A Flow
EGR / emissions
Generic OBD-II
Medium
What it means
P0400 means the ECM has detected a problem with exhaust gas recirculation flow — the system that routes some exhaust back into the intake to cut emissions and combustion temperature. Flow is either too low or too high, usually from carbon build-up or a stuck valve. It can cause rough idle, knock, or a failed emissions test.
Medium
Can I keep driving? Usually drivable — but if the EGR is stuck open you may get rough idle, hesitation or stalling, and a stuck-closed valve can cause light knock; fix it soon.
Common symptoms
What you'll notice
- Check-engine light on
- Rough idle or stalling if the EGR is stuck open
- Light knock/ping or hesitation under load if flow is too low
- May fail an emissions/smog test
Common causes
Most likely first
- Carbon-clogged EGR valve or passages restricting flow (most common)Likely
- EGR valve stuck open or stuck closedCommon
- Faulty EGR solenoid or DPFE/flow sensor misreading flowCommon
- Vacuum leak or broken hose to the EGR valveLess often
- Wiring or connector fault in the EGR circuitLess often
- Faulty PCM (rare)Less often
How to diagnose & fix
Work through it in order
- Read freeze-frame and command the EGR open/closed with a scan tool if supported to see whether flow responds.
- Remove and inspect the EGR valve and passages for heavy carbon build-up; clean or replace as needed.
- Test the EGR solenoid and, where fitted, the DPFE/flow sensor and its vacuum supply.
- Check the EGR wiring, connector, and vacuum hoses for faults.
- Clear the code after repair and confirm the EGR monitor passes on a road test.
FAQ
Quick answers
Is it safe to drive with P0400?+
Short-term driving is usually okay, but a stuck-open valve can stall the engine and a clogged one raises emissions and knock risk, so fix it soon.
Can I just clear P0400?+
It comes back once the ECM tests EGR flow again. Clean or repair the EGR system first, then clear the code.
Related codes
Often seen with P0400
Read & clear P0400 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX