P0135 fault code: O2 Sensor Heater Circuit

Last updated · OBDCode UK editorial

Can I drive with P0135? — Drive with care

You may keep driving if the car feels normal. Get the heater circuit diagnosed before an MOT — an illuminated engine management light is still a fail. Stop if the engine runs poorly or the light flashes.

P0135 means “O2 Sensor Heater Circuit” — a fuel and air metering fault in the powertrain. Drive with care. The heater in the Bank 1, Sensor 1 oxygen sensor is not working as the computer expects. The sensor may be slow to warm; this is a heater-circuit fault, not a mixture or catalyst verdict by itself.

Can I DIY?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.

Caution — conditions applyDIY boundary
ToolsBasic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.

P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0135 mean?

P0135 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 Heater Circuit”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

How to diagnose P0135 yourself

  1. Confirm P0135 and companion codes: Scan with an OBD-II reader. Note whether P0135 is current or pending, freeze-frame, and any other oxygen-sensor or heater codes (for example a downstream heater code). Stop if: You also have a flashing engine management light, misfire codes, or overheating.
    Generic driver footwell showing a broad OBD-II socket search zone and the usual 16-pin socket shape.
    Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim.
  2. View Bank 1 Sensor 1 live data after warm-up: Outdoors, watch Bank 1 Sensor 1 activity and, if your scanner shows it, heater status. Compare with the downstream sensor only as a data check — this does not authorise sensor replacement. Do not idle in a closed garage. Stop if: You would need to work under the car, the exhaust is hot enough to burn, or fumes are in the cabin.

Will P0135 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.

What the MOT data says

These figures are copied from the MOT fail cohort for the certificate wording below. They are not a diagnosis of this car.

Certificate wording: “Oxygen sensor Emission control equipment fitted by the manufacturer missing, obviously modified or obviously defective”.

Of 748 Class 4 vehicles that failed a 2024 MOT on that wording, 0.13% (95% CI 0.0–0.8) failed on the same wording again in 2025. The not-tested gap versus control was 18.1 points.

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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P0135 — quick answers

Is it safe to drive with P0135?

Drive with care. You may keep driving if the car feels normal. Get the heater circuit diagnosed before an MOT — an illuminated engine management light is still a fail. Stop if the engine runs poorly or the light flashes.

Can I fix P0135 myself?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Will P0135 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.