P0172 fault code: System Too Rich

P0172 means “System Too Rich” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Professional diagnosis only — we do not publish a unique check list for P0172 yet.

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 P0172

Do not drive until checked. The computer thinks this bank is running too rich: more petrol than the target mixture, or not enough air. That is a fuel-trim complaint. We do not yet have a verified UK check path unique to P0172.

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

  • System Too Rich: Registration number
  • System Too Rich: Exact code P0172 and any companion codes
  • System Too Rich: Petrol smell, black smoke, or poor running if noticed
  • System Too Rich: Recent work on the intake, fuel system, or sensors
P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0172 mean?

P0172 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P01xx block, which covers fuel and air metering. The stored description is “System Too Rich”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0172 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. Car loses power when accelerating: safe next stepsReduced power is a symptom, not a parts diagnosis; leave fast traffic and preserve warning and code evidence.
P0100Mass or Volume Air Flow Circuit P0101Mass or Volume Air Flow Circuit Range/Performance P0102Mass or Volume Air Flow Circuit Low Input P0103Mass or Volume Air Flow Circuit High Input P0104Mass or Volume Air Flow Circuit Intermittent P0105Manifold Absolute Pressure/Barometric Pressure Circuit P0106Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance P0107Manifold Absolute Pressure/Barometric Pressure Circuit Low Input

P0172 — quick answers

Is it safe to drive with P0172?

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

Will P0172 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.