fault code guide
P0135: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
The internal electric heater in your engine's front oxygen sensor has stopped working properly. Your car will take longer to manage its fuel mixture when cold, which increases emissions and fuel consumption.
How it shows up: The amber engine management light will glow steadily on your dashboard, and the code P0135 is detected using an OBD diagnostic scanner. You may notice slightly higher fuel consumption or rougher running during cold mornings.
Updated 9 November 2026
Can I keep driving?
Service soon
You can safely drive for now, but leaving it unrepaired will waste petrol and can eventually overload the catalytic converter.
What to do right now
- 1Check your instrument cluster to confirm the engine management light is steady amber, not flashing.
- 2Pay attention to how the car drives from a cold start for any hesitation or surging.
- 3Book an inspection or fault scan with an independent garage within the next week or two.
Likely causes
Most likely first.
- Likely
Failed oxygen sensor heating element
The thin internal heating coil inside the front sensor burns out over time through thousands of heating and cooling cycles. Once broken, the engine computer cannot heat the sensor to its operating temperature quickly.
- Possible
Damaged wiring or melted harness
The front sensor wiring runs close to hot exhaust components and moving engine parts. Heat can melt the protective conduit, causing the heater power or earth wire to break or short out.
- Possible
Blown oxygen sensor fuse
A temporary short circuit or failing sensor element can draw excess electrical current and blow the dedicated heater fuse in the engine fuse box.
- Rare
Corroded connector terminals
Water, road salt or engine oil can penetrate the plastic multi-plug connector. Corrosion increases electrical resistance until the control module logs a heater circuit fault.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Inspect the sensor wiring
Wait until the engine is completely cold, open the bonnet, and locate the front oxygen sensor screwed into the exhaust manifold. Trace the cable back to its connector to check for frayed wires, burnt insulation or chafing against metal.
- 2
Check the connector plug
Unclip the sensor electrical plug carefully and look inside both ends. Check that the metal pins are clean, straight and free from green corrosion or oil contamination.
- 3
Check the oxygen sensor fuse
Consult your owner's handbook to find the fuse box diagram under the bonnet and identify the fuse labelled for oxygen sensors or engine management. Pull the fuse and check whether the thin metal strip inside is broken.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total cost depends mainly on whether you choose an original manufacturer sensor or a quality aftermarket brand, and how easily accessible the exhaust manifold is in your engine bay.
Front oxygen sensor replacement
- Do it yourself
- €85–210£75–185 in the UK
- At a garage
- €170–340£150–300 in the UK
Wiring harness repair and connector cleaning
- Do it yourself
- Garage job
- At a garage
- €90–190£80–170 in the UK
Heater circuit fuse replacement and circuit check
- Do it yourself
- €2–10£2–8 in the UK
- At a garage
- €50–100£45–85 in the UK
When to go to a garage
- The engine management light begins to flash or the engine starts misfiring heavily.
- Replacing a blown fuse causes the new fuse to blow immediately when the ignition turns on.
- The oxygen sensor is seized tight in the exhaust manifold and will not budge without risk of stripping threads.
Questions drivers ask
Can I keep driving my car with fault code P0135?
Yes, your car is safe to drive for everyday journeys while you arrange a repair. Once the engine reaches full operating temperature, exhaust gas heat keeps the sensor warm enough to work normally. However, cold-start fuel efficiency and tailpipe emissions will suffer until the heater is fixed.
Will P0135 cause my car to fail the NCT or MOT?
On the UK MOT, a lit engine management light is an automatic fail for petrol cars registered from July 2003 and diesels from July 2008. On the Irish NCT, an engine warning light alone yields an advisory pass, but the car will fail if sluggish sensor warming causes your tailpipe exhaust emissions to exceed legal limits during testing.
How does P0135 differ from P0141?
Both codes refer to an oxygen sensor heater fault, but they point to different locations on your exhaust. P0135 targets Sensor 1 on Bank 1, which sits upstream before the catalytic converter to regulate the fuel mix. P0141 refers to Sensor 2, which sits downstream after the catalytic converter to monitor exhaust cleaning efficiency.
Will clearing the fault code fix the issue permanently?
No, simply clearing the code with a scan tool will only turn off the light temporarily. The engine control unit tests the heater circuit resistance every time you start the car from cold. As soon as the computer detects an open circuit or excessive resistance again, the light and code will return.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.