What does the heater in an oxygen sensor do?

It brings the sensor up to its working temperature quickly. A zirconia oxygen sensor produces no usable signal until its ceramic element is several hundred degrees Celsius, and waiting for exhaust gas alone to achieve that would take minutes. A built-in electric heater does it in seconds, so the engine can start controlling its mixture properly almost immediately after a cold start. Since cold starts produce a disproportionate share of a petrol car's emissions, that few seconds matters.

Will P0135 fail an MOT?

Yes, on the lamp. An illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, and the car fails on that alone. The gas readings will usually pass without difficulty, because the tester warms the engine fully before sampling and exhaust heat brings the sensor up to temperature regardless of whether its heater works. It is a clear example of a car failing while producing clean emissions at the moment of measurement.

Can I just replace the heater instead of the whole sensor?

No. The heating element is built into the sensor body during manufacture and is not a separately available part, so a failed heater means a new sensor even though the measuring element may be working perfectly. The one thing worth checking before spending anything is the fuse and relay supplying the heater circuit, since a dead supply produces the same code and costs almost nothing to rule out — especially if more than one heater code is stored at once.

Is it safe to drive with a P0135 code?

Yes. Nothing about the way the car drives, accelerates or stops is affected, and no damage is accumulating. The practical cost is confined to short journeys, where the engine stays in its warm-up fuelling mode for longer than it should, using slightly more fuel and producing more emissions until the exhaust warms the sensor by itself. The car will not pass an MOT while the lamp is lit, which is the real reason to fix it.

Powertrain code · standard

P0135Heater inside the front oxygen sensor has failed

Standard definition: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)

Last updated 27 July 2026

Quick answer

Oxygen sensors contain a small electric heater so they can start working within seconds of a cold start instead of waiting for the exhaust to warm them. The control unit has found that heater's circuit faulty. The sensor's ability to read the exhaust may be perfect — it is the little heating element that has gone, and on most cars that still means a whole new sensor.

How the car works this out

A zirconia oxygen sensor produces no usable signal until its element reaches several hundred degrees Celsius, so a resistive heater is built into it to get there in seconds rather than minutes. That heater is on a completely separate circuit from the signal wires. The control unit monitors the current the heater draws, or the resistance of its circuit, and compares it against the expected range. An open circuit, a short, or a current draw well outside the normal window sets this code. It says nothing at all about whether the sensor can measure exhaust oxygen.

What you might notice

Nothing you would feel — this is a code found by reading, not by driving
Slightly worse fuel consumption on short journeys, since the engine stays in its open-loop warm-up mode longer
Higher emissions in the first few minutes of a journey
On some cars, other heater circuit codes appearing at the same moment

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The heater element inside the sensor has gone open circuit, which is ordinary end-of-life failureCommon
How to check: With the sensor unplugged, measure the resistance across the two heater terminals. A working element reads a low resistance of a few ohms; an open circuit reads infinite and confirms the sensor.
2A blown fuse feeding the heater circuit, which on many cars is shared with other sensors so several heater codes appear at onceCommon
How to check: Check the fuse first. It takes a minute, it costs nothing, and several heater codes stored simultaneously is a strong hint that the common supply rather than the sensors is at fault.
3A corroded connector or a heater wire chafed against the exhaust or a heat shieldPossible
How to check: Inspect the plug for green corrosion and the loom for hardened, cracked or melted insulation where it passes close to hot metal.
4A failed relay supplying the heater circuitPossible
5A failed driver circuit inside the control unit, which is the least likely and last thing to suspectLess common

What to do about it

1Check the fuse and the relay before ordering a sensor. It is the cheapest possible outcome and it is a real one, particularly where more than one heater code is stored.
2Measure the heater resistance with the sensor unplugged. An open circuit at the sensor end is conclusive and saves any further hunting.
3Inspect the connector and the loom. These sensors sit in the hottest, dirtiest part of the car and the wiring fails almost as often as the element.
4Accept that a failed heater usually means a new sensor. The heater is built into the sensor body and is not separately serviceable, even though the measuring element may be in perfect order.
5Fit a sensor with the correct heater specification. A substitute with different heater resistance can set the same code again even when everything is physically sound.

Can you keep driving?

Yes

This fault does nothing to how the car drives, and on a long journey it has essentially no effect at all, because the exhaust eventually warms the sensor anyway. It matters on short trips, where the engine may never leave its warm-up fuelling mode, so economy suffers and emissions are higher than they should be. Nothing here is urgent from a mechanical or safety point of view.

Will it fail an MOT?

This code separates the two halves of the MOT emissions test very cleanly. The gas readings are usually fine, because a tester runs the engine up to full operating temperature before taking a sample, by which point exhaust heat has brought the sensor up to temperature whether its heater works or not. So the printout looks healthy. The failure is the lamp, which is a major defect on petrol cars first used on or after 1 July 2003, and there is no route around it. There is one further consideration: the oxygen sensor heater has its own readiness monitor on many vehicles, and it cannot complete while the circuit is faulty, so clearing the code before the test leaves a monitor permanently not ready and can prevent the emissions check being completed at all.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

What does the heater in an oxygen sensor do?: It brings the sensor up to its working temperature quickly. A zirconia oxygen sensor produces no usable signal until its ceramic element is several hundred degrees Celsius, and waiting for exhaust gas alone to achieve that would take minutes. A built-in electric heater does it in seconds, so the engine can start controlling its mixture properly almost immediately after a cold start. Since cold starts produce a disproportionate share of a petrol car's emissions, that few seconds matters.
Will P0135 fail an MOT?: Yes, on the lamp. An illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, and the car fails on that alone. The gas readings will usually pass without difficulty, because the tester warms the engine fully before sampling and exhaust heat brings the sensor up to temperature regardless of whether its heater works. It is a clear example of a car failing while producing clean emissions at the moment of measurement.
Can I just replace the heater instead of the whole sensor?: No. The heating element is built into the sensor body during manufacture and is not a separately available part, so a failed heater means a new sensor even though the measuring element may be working perfectly. The one thing worth checking before spending anything is the fuse and relay supplying the heater circuit, since a dead supply produces the same code and costs almost nothing to rule out — especially if more than one heater code is stored at once.
Is it safe to drive with a P0135 code?: Yes. Nothing about the way the car drives, accelerates or stops is affected, and no damage is accumulating. The practical cost is confined to short journeys, where the engine stays in its warm-up fuelling mode for longer than it should, using slightly more fuel and producing more emissions until the exhaust warms the sensor by itself. The car will not pass an MOT while the lamp is lit, which is the real reason to fix it.

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