P0400 Code: EGR Flow Malfunction — Causes & Fix
P0400 means a general fault in exhaust gas recirculation (EGR) flow — the EGR system isn't flowing correctly.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $40 – $600
- Time to fix
- 30 minutes – 2 hours
What it means
Causes include a clogged EGR valve or passages (carbon), a stuck valve, or a control fault. P0400 is a generic OBD-II powertrain code defined in the SAE J2012 standard, so it applies to virtually any 1996-or-newer petrol car or light truck regardless of make. Fix the underlying fault, then clear the code with an OBD-II scan tool — it will return if the problem isn't resolved.
Symptoms
- Check engine light on
- Rough idle or pinging
- May fail emissions
How to fix it
- 1
Clean/inspect the EGR valve
1. Remove and clean carbon from the EGR valve and passages. 2. Confirm the valve opens and closes.
- 2
Check the control
1. Test the EGR control/solenoid. 2. Repair, then clear the code.
Frequently asked questions
Is carbon the usual cause of P0400?
Very often — carbon build-up in the EGR valve or passages restricts flow. Cleaning frequently resolves it.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.