P0155 Code – O2 Sensor Heater Circuit Malfunction

The P0155 diagnostic trouble code indicates that the Engine Control Unit (ECU) has detected a malfunction in the heater circuit of the oxygen (O2) sensor on Bank 2 Sensor 1.

The heater element inside the O2 sensor helps it reach optimal operating temperature quickly after engine start. Without proper heating, the sensor responds slowly or may not function correctly.

When the ECU detects that the heater circuit is not operating within expected parameters, it stores the P0155 code.

Symptoms of Code P0155

Common symptoms include:

• Check Engine Light illuminated

• Increased fuel consumption (especially cold start)

• Delayed closed-loop operation

• Slight rough running when engine is cold

• Higher emissions during warm-up

Common Causes of P0155

Typical causes include:

• Faulty O2 sensor heater (Bank 2 Sensor 1)

• Blown fuse related to heater circuit

• Open or shorted wiring

• Corroded or loose connectors

• Faulty relay (if equipped)

• ECU fault (rare)

How Serious Is Code P0155

The P0155 code is considered a low to moderate severity issue.

The vehicle remains drivable, but:

• engine efficiency during cold start is reduced

• emissions increase

• fuel consumption may rise

Repair is recommended but not urgent.

How to Diagnose Code P0155

Diagnosis typically involves:

1. Scanning the vehicle with an OBD2 tool

2. Checking heater circuit resistance

3. Verifying power and ground supply

4. Inspecting wiring and connectors

5. Checking related fuses and relays

6. Monitoring sensor warm-up behavior

A faulty heater circuit confirms the issue.

How to Fix Code P0155

Common repair solutions include:

• Replacing the O2 sensor (Bank 2 Sensor 1)

• Replacing blown fuses

• Repairing wiring or connectors

• Replacing faulty relay

• Clearing the fault code after repair

In most cases, replacing the sensor resolves the issue.

Vehicles Commonly Affected

The P0155 code may appear on vehicles with multiple cylinder banks, including:

• BMW

• Mercedes-Benz

• Audi

• Ford

• Toyota

• Nissan

FAQ – P0155 Code

Can I drive with code P0155?

Yes, but fuel efficiency during cold starts may be reduced.

What does the heater circuit do?

It helps the O2 sensor reach operating temperature quickly.

Is this a common issue?

Yes, especially on higher mileage vehicles.