P0400 fault code: Exhaust Gas Recirculation Flow

P0400 means “Exhaust Gas Recirculation Flow” — an auxiliary emission controls fault in the powertrain.

Can I DIY?

Professional-only boundary. No code-specific DIY path yet — confirm the code if you wish, then book a garage.

Do not DIYReviewed page boundary
ToolsSpecialist equipment

This is the conservative boundary from the reviewed code record. This page has no frozen site-wide safety grade, so it does not publish a repair route.

What to tell the garage about P0400

Do not drive until checked. The computer has logged an exhaust-gas recirculation (EGR) flow fault, without saying whether flow is too low or too high. We do not yet have a verified safety row for this exact code, so treat it as garage work.

Take these reviewed, code-specific details with you. They help the garage confirm the fault without turning this page into a repair procedure.

  • Exhaust Gas Recirculation Flow: Registration number
  • Exhaust Gas Recirculation Flow: Exact code P0400 and any other codes from the same scan
  • Exhaust Gas Recirculation Flow: Whether the engine management light is steady or flashing
  • Exhaust Gas Recirculation Flow: That the EGR valve has not been removed or cleaned at home
P04xxAuxiliary emission controls
MOT fail riskif the warning light stays on

What does P0400 mean?

P0400 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P04xx block, which covers auxiliary emission controls. The stored description is “Exhaust Gas Recirculation Flow”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0400 fail an MOT?

An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off.

Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.

Related guides

Car diagnostic test cost: compare the scopeUK booking marketplaces currently publish diagnostic averages of about £52–£65. Engine management light on: steady or flashing?A steady engine management light means the computer has stored a fault. Drive with caution and get the code read. A flashing light is different — stop when it is safe.
P0401Exhaust Gas Recirculation Flow Insufficient Detected P0402Exhaust Gas Recirculation Flow Excessive Detected P0403Exhaust Gas Recirculation Control Circuit P0404Exhaust Gas Recirculation Control Circuit Range/Performance P0405Exhaust Gas Recirculation Sensor 'A' Circuit Low P0406Exhaust Gas Recirculation Sensor 'A' Circuit High P0407Exhaust Gas Recirculation Sensor 'B' Circuit Low P0408Exhaust Gas Recirculation Sensor 'B' Circuit High

P0400 — quick answers

Is it safe to drive with P0400?

Do not drive until checked. Driveability guidance is not available for this code — treat as do not drive until checked.

Will P0400 fail my MOT?

An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off.

Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.