P0179 fault code: Fuel Composition Sensor Circuit High

P0179 means “Fuel Composition Sensor Circuit High” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Garage only: a high composition-circuit reading is not a tank drain DIY.

Do not DIYReviewed page boundary

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 P0179

Do not drive. The fuel-composition sensor circuit is sitting higher than the computer allows. Treat it as an electrical-range fault on the composition sender, not as oil-temperature or rail-pressure.

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

  • Fuel Composition Sensor Circuit High: Number plate
  • Fuel Composition Sensor Circuit High: Exact code P0179
  • Fuel Composition Sensor Circuit High: Dash messages about fuel type or ethanol, if shown
P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0179 mean?

P0179 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 “Fuel Composition Sensor Circuit High”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0179 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.
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

P0179 — quick answers

Is it safe to drive with P0179?

Do not drive. P0179 has no verified driving guidance. Do not drive until a workshop has checked the high circuit. Do not assume 'high' means too much ethanol and keep using the car.

Will P0179 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.