P0178 fault code: Fuel Composition Sensor Circuit Low

P0178 means “Fuel Composition Sensor Circuit Low” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Professional only: a low composition-circuit voltage is not a home fix.

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 P0178

Do not drive. The fuel-composition sensor circuit is sitting lower than the computer allows. That points at wiring, earth or a weak sender output — not at a mixture that is already too rich.

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 Low: Number plate
  • Fuel Composition Sensor Circuit Low: Exact code P0178
  • Fuel Composition Sensor Circuit Low: Whether the engine starts, stalls, or will not rev
P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0178 mean?

P0178 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 Low”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0178 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.
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P0178 — quick answers

Is it safe to drive with P0178?

Do not drive. Without verified driving guidance, P0178 is treated as do not drive until a garage traces the low circuit. A low reading is not proof the tank is full of the wrong fuel, and it is not a rail-pressure low code.

Will P0178 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.