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Home > Fault Codes > P0135

P0135 — O2 Sensor Heater Circuit (Bank 1 Sensor 1)

By Mr Auto Fixer — Professional Mechanic, 20+ Years Experience

The internal heating element of the O2 sensor has failed or is not reaching operating temperature. The sensor takes longer to warm up.

Medium — Fix Soon
Last checked: May 2026

What Is P0135?

P0135 means the ECU has detected a fault in the heater element circuit of the upstream oxygen sensor on Bank 1. Modern O2 sensors contain a built-in ceramic heater that brings the sensor to operating temperature (300–800°C) within 30 seconds of startup, allowing closed-loop fuel control to begin quickly. Without the heater working, the sensor relies on exhaust heat alone — taking several minutes to reach temperature.

The heater circuit is separate from the sensing element circuit. P0135 specifically flags the heater — the sensing element itself may still work. However, without the heater, cold starts run in open-loop for longer than designed, increasing emissions and fuel consumption. The code also often precedes or accompanies P0133 (slow response) because a cold sensor always responds slowly.

Common Symptoms

  • Engine management light on
  • Extended open-loop cold start period
  • Increased fuel consumption, particularly on short journeys
  • Possible P0133 (slow response) stored alongside
  • Usually no dramatic driveability issues at operating temperature

Common Causes

Failed O2 Sensor Heater ElementThe internal heater element wire has broken or burnt out, creating an open circuit. Most common cause, particularly on sensors over 80,000 miles.
Blown Heater FuseThe O2 sensor heater circuit is typically fused. A blown fuse prevents voltage reaching the heater. Check fuse boxes for any blown fuses related to the O2 sensor or lambda heater circuit.
Faulty Heater RelaySome vehicles route O2 sensor heater power through a relay. A failed relay prevents power reaching the heater circuit.
Wiring Short or Open CircuitThe wiring from the fuse/relay to the sensor heater has broken or shorted, preventing current flow through the heater element.
ECU Heater Control FaultThe ECU controls the heater circuit output on many modern vehicles. Rarely, the ECU output transistor fails. Confirmed only after all other checks are clear.

How to Diagnose P0135

1

Measure Heater Voltage

With the ignition on (or engine running), probe the two heater wires at the O2 sensor connector. Battery voltage (12–14V) should be present on at least one heater pin when the ECU activates the heater circuit. No voltage indicates a blown fuse, failed relay, or open wiring.

2

Measure Heater Resistance

Disconnect the sensor and measure resistance across the heater element pins (the two pins that showed voltage). Typical resistance is 2–10 ohms depending on sensor type. Infinite resistance (OL) confirms the heater element has failed. Zero ohms indicates a short circuit.

3

Check the O2 Sensor Heater Fuse

Locate the fuse associated with the lambda or O2 sensor heater in the main fuse box. Check it with a test light or multimeter — power should be present on both sides with the ignition on. Replace any blown fuse and check whether it blows again immediately (indicating a short).

4

Inspect Wiring from Fuse to Sensor

Trace the heater wiring from the fuse box to the sensor. Look for heat damage (the loom often runs near the exhaust), chafing, or melted insulation. Repair any damage found.

5

Replace the O2 Sensor

If fuse and wiring are fine but heater resistance is out of specification, replace the sensor. The heater element cannot be replaced separately. Sensor cost: £40–£120.

Verdict

Start by checking the heater fuse and measuring heater resistance at the sensor. An open-circuit heater element (infinite resistance) confirms sensor replacement is needed. A blown fuse indicates a wiring short that must be found before fitting a new sensor. Total repair cost is typically £60–£150.

Mr Auto Fixer
Written by
Mr Auto Fixer
Qualified Mechanic20+ Years ExperienceUK Based

Professional UK mechanic with over 20 years of hands-on experience. All guides are based on real workshop repairs — not theory.

About Mr Auto Fixer
Common Questions

Frequently Asked Questions

It means the heater element inside the sensor has failed — not necessarily the sensing element itself. However, the heater and sensor are inside the same unit and cannot be separated, so replacement of the complete sensor is required.
Yes, the engine will still run. But fuel economy will be worse on cold starts and short journeys, as the ECU stays in open-loop for longer without a heated sensor reaching temperature quickly. Repair within a few weeks.
Most modern O2 sensors have four wires: two for the heater circuit and two for the sensor signal. The heater wires are usually thicker gauge and are powered from the vehicle’s 12V supply. Sensor signal wires are typically thinner. Refer to wiring diagrams for your specific vehicle.
If the fault is the heater element in the sensor, yes. If the fault is a wiring break, blown fuse, or relay problem, a new sensor will not fix it. Confirm the heater circuit is delivering voltage to the sensor before blaming the sensor itself.
It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.
Related

Related Fault Codes

P0130
O2 Sensor Circuit — general circuit fault
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P0133
O2 Sensor Slow Response — often caused by failed heater
Read Guide →
P0136
O2 Sensor Circuit Bank 1 Sensor 2 — downstream sensor fault
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