P0140 fault code: O2 Sensor Circuit No Activity Detected

P0140 means “O2 Sensor Circuit No Activity Detected” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. You can read P0140; an inactive rear probe is garage electrical work.

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 P0140

Do not drive. The downstream oxygen probe on bank 1 shows no activity. The computer is missing the rear exhaust-oxygen signal it uses to judge catalyst work on that bank.

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: Registration number
  • O2 Sensor Circuit No Activity Detected: Exact code P0140 and any companion codes
  • O2 Sensor Circuit No Activity Detected: Whether the engine management light is steady or flashing
  • O2 Sensor Circuit No Activity Detected: Whether the rear bank-1 sensor wiring looks damaged from the side of the car (no crawl)
P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0140 mean?

P0140 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 P0140 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

P0140 — quick answers

Is it safe to drive with P0140?

Do not drive. We have no verified driving guidance for an inactive bank-1 rear oxygen circuit. Do not drive until a garage confirms the code. A silent after-cat probe is not a crawl-under replacement guide.

Will P0140 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.