P0400 EGR flow malfunction
P0400 keeps returning because the ECU retests EGR flow on almost every journey, and a fault in the valve, pipework or sensors is still there after a reset. We read the live EGR data at your home, yard or depot and tell you what's really behind it.
Why P0400 comes back after clearing
The official wording is EGR flow malfunction, a broad code meaning the flow didn't match what the ECU expected, without saying whether it was too much or too little. A code reader resets the light but doesn't touch the valve or pipes.
Once the engine is warm and running steadily, the ECU opens the valve and watches for the change in air flow or intake pressure. If the change is wrong, P0400 is stored again, and the cycle carries on until the cause is found.
The usual causes behind P0400
| Cause | How it's checked |
|---|---|
| Carbon clogging the EGR valve or passages | Commanded and actual valve position, plus air flow change when it opens |
| Valve stuck open or shut | Actuation test on live data |
| EGR cooler blocked or bypass flap sticking | Temperature readings and cooler-related codes |
| Air flow or intake pressure sensor reading wrong | Sensor figures compared with engine load |
| Vacuum or electrical control fault | Vacuum test, circuit checks and codes such as P0403 |
Symptoms
- An engine light that comes back a day or two after clearing
- A rough or uneven idle
- Pinking or a harsher knock at light throttle on some engines
- Black smoke or a sooty exhaust when the valve sticks open
- No change at all in how it feels, on plenty of vehicles
Using the vehicle with P0400 showing
Most vehicles keep running normally with P0400 stored, but the fault behind it rarely stays still. A valve sticking open adds soot to the intake and DPF, and one sticking shut raises combustion temperatures. The lit light is an MOT fail either way.
Vans and fleets with P0400
Vans doing short hops with lots of idling build carbon fastest, so P0400 can crop up on several vans in the same fleet. We test them at your yard or depot between runs, with no garage trip needed to find the fault.
A trade account suits businesses that book us regularly, and you can find out more on our trade and partners page.
How we stop P0400 coming back
EGR flow diagnosis, £100 + VAT
We test the valve, read the flow and sensor figures and check for a blocked cooler, then explain what's causing the fault and what will fix it.
Valve or cooler replacement
A garage replaces the faulty valve or cooler. Our EGR valve replacement cost guide gives an idea of the cost.
EGR delete, £220 + VAT
A software delete turns off EGR control and the flow test with it.
Reviews
Why does P0400 come back after it's been cleared?
Because clearing doesn't change the flow. The ECU's next EGR test meets the same carbon, sticking valve or sensor error and logs P0400 once more.
Can a remap cause P0400?
Only a poorly written one that changes air or boost targets without allowing for EGR. A carefully set-up remap keeps the EGR flow test working as the manufacturer intended.
Can another fault set off P0400?
Yes. An intake leak, a faulty air flow sensor or a boost problem can all upset the figures the ECU uses to judge EGR flow. Codes like P0402 can appear alongside it.
Can you check P0400 on our vans at the depot?
We can. Tell us which vans are showing it and when they're back at base, and we'll test them on site.
What should I tell you when I book for P0400?
The registration, the full code, how quickly it comes back after clearing and whether there's any stalling, smoke or loss of power.
Can a vehicle with the EGR deleted still be used on the road?
No. We carry out the EGR delete for off-road and export use only, and our guide on whether an EGR delete is legal explains why.
Stop P0400 coming back
Book a fault check and we'll find the cause of the EGR flow fault on your driveway, at work or at the depot.
More fault codes explained
Eccles, Manchester M30 0SF