General EGR flow fault — could be too much or too little flow.
P0400 — also written PO4OO, P 0400, 0400 — is the OBD-II code for egr flow malfunction. Same code, same fix: this page covers every spelling of it.
EGR flow malfunction — the ECU has detected the exhaust-gas-recirculation valve is not moving to the position it was commanded to, or the flow through the EGR circuit is outside expected limits. On UK cars this is almost always carbon jamming the valve, a failed position sensor, or a snapped internal motor. P0400 is broader than P0401/P0402, so a proper test is essential before buying parts.
P0400 is a emissions fault code, typically costing £150–£800 to put right in a UK garage.
What P0400 actually feels like to drive
- Engine management light on with no obvious running fault
- Rough or lumpy idle, sometimes stalling at junctions
- Hesitation or flat spot when pulling away from low RPM
- Slight increase in fuel consumption over a tankful
- Black smoke on hard acceleration when the valve is stuck open
- Paired codes: P0401 (insufficient flow), P0402 (excessive flow), P0403 (circuit), P0404 (range/performance)
What causes P0400?
- Carbon-blocked EGR
- Failed EGR valve
- Cooler blocked
UK vehicles that throw P0400 most often
- Vauxhall Astra H/J, Insignia, Zafira 1.7/2.0 CDTi — EGR valve sticks open or closed
- Ford Focus / Mondeo / C-Max 1.6/1.8/2.0 TDCi — motorised EGR fails around 80k
- VW Golf / Passat / Audi A3 / A4 2.0 TDI PD and CR — EGR cooler and valve carbon
- BMW 320d / 520d N47/B47 — EGR valve position sensor fault
- Peugeot 308 / 3008 / Citroën C4 1.6 HDi — EGR motor or cooler blocked
- Nissan Qashqai / X-Trail 1.5/1.6 dCi — EGR valve and cooler soot issues
How P0400 is fixed
- Clean or replace EGR valve
- Descale EGR cooler
The P0400 misdiagnosis that wastes money
Because P0400 is a generic 'flow malfunction' code, garages often replace the EGR valve first and ask questions later. That's a £180–£420 mistake on many cars. The right test order: one — read live EGR position sensor data and compare 'commanded' vs 'actual' while the engine is warm; if they track together the valve is moving and the fault is likely the differential-pressure sensor or a blocked cooler. Two — remove and manually check the valve pintle; if it is stuck with carbon, a professional clean can restore it for a fraction of the replacement cost. Three — check the wiring and connector; Vauxhall 1.7 CDTi and Ford TDCi EGR connectors are notorious for pin corrosion and broken wires.
What a UK garage should charge for P0400
UK independent garage: diagnostic + live EGR position test £50–£90, EGR valve removal and chemical clean £120–£220 fitted, EGR valve replacement £180–£420 fitted (Vauxhall/Ford common diesels), EGR cooler replacement £320–£680, wiring/connector repair £60–£180. Main dealer: typically 40–60% more. Labour From £70/hr at Bob's.
P0400 repair cost in the UK
Most P0400 repairs land between £150–£800 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0400
Bob's 32 years on the tools rule for P0400: never authorise an EGR valve swap until you've seen live data showing the valve genuinely can't reach commanded position. On Vauxhall 1.7 CDTi the valve itself is fine 70% of the time — it's the 5-pin connector behind the engine that corrodes and gives the ECU a false 'malfunction' reading. Five minutes with a contact-cleaner spray and dielectric grease has fixed dozens. On Ford 2.0 TDCi a stuck-open EGR often causes a lumpy idle but smooth motorway cruising; if that matches your symptoms, ask for a clean before a replacement. On VW/Audi 2.0 TDI the cooler blocks with carbon and the valve gets the blame — always pressure-test or visually inspect the cooler. And if your car is out of warranty, a warranty-safe EGR blanking plate is not the answer for a daily driver in the UK; it will fail the MOT emissions test and can cause other codes. Want a second opinion before spending money? <a href="/ask-a-mechanic-uk?code=P0400">Ask Bob about your P0400</a> for £14.99 with make, model, mileage and any paired codes — you'll get the right test order first.
Can you drive with P0400?
Yes but MOT emissions risk.
Urgency: medium.