Fault codeP0155

MODERATE

O2 Sensor Heater Circuit Malfunction (Bank 2 Sensor 1)

What this code means

O2 Sensor Heater Circuit Malfunction (Bank 2 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.

P0155 is a generic code: it means the same thing on every make. P0155 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit has detected an electrical anomaly on this circuit without saying exactly which. Diagnosis starts with a visual check of the wiring and connector, then measuring the component — resistance, supply, ground — with a multimeter.

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.

What you notice

Usually nothing while driving. The mixture stays less precise for the first minutes after a cold start, so emissions and consumption rise slightly, and the car may fail the emissions test.

How to check it

The heater is part of the sensor: when it fails, the whole sensor is replaced. Before that, measure the heater's resistance across its two dedicated pins (usually a few ohms), check the fuse that feeds it and inspect the connector, which often suffers from heat and water.

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 →

How to read this code

PPowertrain
0standardised code
1Fuel and air metering
55P0155