P0166 fault code: O2 Sensor Circuit No Activity Detected

Can I drive with P0166? — Do not drive

We have no verified driving guidance for P0166, so treat the car as not to be driven until a garage confirms the sensor circuit. A dead oxygen-sensor signal is not a licence to keep using the engine in open loop.

P0166 means “O2 Sensor Circuit No Activity Detected” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Professional only: confirm P0166 on a scanner, then hand the car to a garage.

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 P0166

Do not drive. The control unit is not seeing a changing oxygen-sensor voltage. Without that swing it cannot tell rich from lean, so closed-loop fuel trim on that sensor is unavailable.

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

O2 Sensor Circuit No Activity Detected: Number plate
O2 Sensor Circuit No Activity Detected: Exact code P0166 and any companion oxygen-sensor codes
O2 Sensor Circuit No Activity Detected: Whether the engine management light is steady or flashing

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Fault code: P0166 — O2 Sensor Circuit No Activity Detected.
Can I drive: Do not drive. We have no verified driving guidance for P0166, so treat the car as not to be driven until a garage confirms the sensor circuit. A dead oxygen-sensor signal is not a licence to keep using the engine in open loop.
Question: What does this code mean for a UK car, is it safe to drive, what are the most likely causes, and what should I ask a garage to check before replacing parts?

For your garage

REPAIR BRIEF — prepared by the owner with obdcode.co.uk
Fault code: P0166 — O2 Sensor Circuit No Activity Detected.
Drive guidance from obdcode.co.uk: Do not drive. We have no verified driving guidance for P0166, so treat the car as not to be driven until a garage confirms the sensor circuit. A dead oxygen-sensor signal is not a licence to keep using the engine in open loop.
Owner’s request: please diagnose before replacing parts, itemise diagnosis time separately, and quote before any work beyond diagnosis.

P01xxFuel and air metering

MOT fail riskif the warning light stays on

What does P0166 mean?

P0166 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 “O2 Sensor Circuit No Activity Detected”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0166 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.

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

P0166 — quick answers

Is it safe to drive with P0166?

Do not drive. We have no verified driving guidance for P0166, so treat the car as not to be driven until a garage confirms the sensor circuit. A dead oxygen-sensor signal is not a licence to keep using the engine in open loop.

Will P0166 fail my MOT?