P0135 — O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
P0135 is about the heater inside the sensor, not the sensor reading. It usually means the sensor needs replacing, and until it is, the engine runs on estimated fuelling for the first minutes of every trip.
The vehicle drives normally. Expect worse economy on short journeys, and readiness monitors that will not complete for an emissions test. This code will cause an emissions test failure.
What P0135 means
An oxygen sensor only reports usefully once it is hot, so each one contains a heater that brings it up to temperature within seconds of a cold start rather than waiting for the exhaust. The module monitors the current that heater draws and sets P0135 when it is outside the expected range. Until the sensor is at temperature the module cannot use its signal, so it runs open loop on pre-programmed values — which costs fuel and emissions on every short trip.
| System | Emission control |
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Symptoms you may notice
What causes P0135
Heater element inside the sensor has failed
The element is a resistance heater sealed inside the sensor body and cannot be repaired. It fails open with age and heat cycling, and this is the cause in the large majority of cases.
Fuse supplying the heater circuit blown
One fuse often feeds several sensor heaters. If more than one heater code is stored at once, check the fuse before condemning any sensor — it is the cheapest possible repair.
Common misdiagnosis
Multiple heater codes at once point at a shared fuse or relay, not at several sensors failing on the same day.
Wiring or connector damaged
The harness runs close to the exhaust. Insulation hardens and chafes, and connectors corrode where they sit under the vehicle. Inspect before replacing the sensor.
Poor ground on the heater circuit
A corroded ground point raises circuit resistance enough to put current draw out of range while everything else appears intact.
How to diagnose it
Check for other heater codes
Scan for all stored codes. Several heater codes together point at a shared supply rather than at multiple failed sensors.
Check the fuse feeding the sensor heaters
Identify the fuse for the oxygen sensor heater circuit and test it in place with a meter rather than by eye — a cracked element can look intact.
Measure heater resistance at the sensor connector
Unplug the sensor and measure across the two heater pins. An open circuit confirms the element has failed. Compare against the specification for the vehicle.
Safety
Let the exhaust cool before working near it.
Check supply voltage and ground at the connector
With the ignition on, confirm battery voltage on the supply pin and a good ground on the return. Wiggle the harness while watching for the reading to drop.
Related codes
Common questions
Can I drive with P0135?
Yes. The engine runs normally, though fuel economy suffers on short trips because the module cannot use the sensor until it warms up on its own.
Which sensor is bank 1 sensor 1?
The sensor before the catalytic converter, on the bank containing cylinder 1. Fitting a sensor in the wrong position is a common and avoidable error — confirm the layout for your engine first.
Can the heater be replaced separately?
No. It is sealed inside the sensor body, so a failed heater means a new sensor.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.