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.

Real-world scan data

Seen in

1,640

scans in the past year

Found in

0.3%

of scans

0.5%

of scans that found faults

Appears alongside · share of P0400 scans

P0683in 25.2% of scans

P0110in 24.9% of scans

P0100in 21.5% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

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.

Read & clear P0400 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX