P0138 fault code: O2 Sensor Circuit High Voltage

P0138 means “O2 Sensor Circuit High Voltage” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. No verified home checks for P0138 — book a garage to read the oxygen-sensor circuit.

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 P0138

Do not drive. An oxygen sensor is sending a voltage higher than the engine computer will accept, so the computer cannot use that exhaust-oxygen reading to trim fuel. We do not have a verified safety row for this code.

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 High Voltage: Registration number
  • O2 Sensor Circuit High Voltage: Exact code P0138 and any companion oxygen-sensor or fuel-trim codes
  • O2 Sensor Circuit High Voltage: Whether the engine management light is steady or flashing
  • O2 Sensor Circuit High Voltage: Any exhaust smell inside the cabin
P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0138 mean?

P0138 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 High Voltage”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0138 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

P0138 — quick answers

Is it safe to drive with P0138?

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

Will P0138 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.