Fault codeP0135

MODERATE

O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

What this code means

O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

“Sensor 1” is the oxygen sensor upstream of the catalytic converter, the one that regulates the mixture; “Sensor 2” sits downstream and monitors the converter.

P0135 is a generic code: it means the same thing on every make. P0135 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Oxygen sensor heater

An oxygen sensor only works once it is very hot, around 300 °C or more. A small heating element built into the sensor brings it to temperature in a few seconds after a cold start, rather than waiting for the exhaust to heat it. The control unit powers it and monitors its current.

Symptoms

No driving symptom in most cases. The oxygen sensor is electrically heated to reach working temperature quickly; without the heater it takes several minutes to give a useful signal, and the control unit stays in fixed mixture meanwhile — slightly higher consumption when cold, and the light on.

Common causes

Burnt-out sensor heater element — the classic failure of a sensor at the end of its life
Blown sensor heater circuit fuse
Sensor connector corroded or melted by exhaust heat
Heater wire cut or chafed
Failed relay or control unit driver stage (rare)

How to diagnose it

First check the sensor circuit fuse — it often feeds several sensors, and if all of them report the same fault, that is it. Then unplug the sensor and measure the resistance between the two heater wires, often the same colour: generally 3 to 15 ohms; infinite, the element is burnt out.

Finally check for 12 V at the loom side of the connector, ignition on or engine running depending on the design.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Coolant temperature
Short term fuel trim bank 1
Long term fuel trim bank 1

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

Fuse: under a euro. Oxygen sensor: €50 to €200 for the part, €80 to €250 fitted.

How to read this code

PPowertrain
0standardised code
1Fuel and air metering
35P0135