P0155 means there’s a problem in the O2 sensor heater circuit for Bank 2, Sensor 1 (the upstream oxygen sensor on the bank that does not contain cylinder #1).

What the code means 

The heater inside the O2 sensor is not working correctly

The ECU expects the sensor to heat up quickly

If it doesn’t see the correct current/voltage, it sets P0155

This affects cold starts, fuel trim, and emissions, and can cause poor MPG.

Location clarification

Bank 2 = side of the engine opposite cylinder #1

Sensor 1 = upstream, before the catalytic converter

Common causes

1. Failed O2 sensor heater (very common)

2. Blown heater fuse

3. Open or shorted wiring to the O2 sensor

4. Poor ground for heater circuit

5. ECU driver issue (rare)

Step-by-step diagnostic process (shop-friendly)

1. Visual inspection (don’t skip)

Check connector for:

Melted plastic

Oil contamination

Bent or corroded pins

Inspect wiring near exhaust for heat damage

2. Check heater power (key ON, engine OFF)

Most O2 heaters use battery voltage

Unplug O2 sensor

Identify heater wires (usually same color, often white)

Use multimeter:

One side should have ~12V

Other side goes to ECU ground control

❌ No voltage → check fuse / relay / wiring

3. Check heater resistance (sensor unplugged)

Measure resistance across heater pins

Typical values:

3–15 ohms (varies by manufacturer)

❌ Infinite or very high resistance → bad O2 sensor ❌ 0 ohms → shorted heater

4. Check ground control (advanced)

With engine running cold:

ECU should pulse ground to heater

Use test light or scope if available

❌ No ground control → wiring or ECU issue

Most common real-world fix

👉 Replace Bank 2 Sensor 1 O2 sensor

⚠️ Important:

Use OEM or high-quality brand

Cheap sensors cause repeat comebacks

Clear code and verify heater readiness monitor

Customer explanation (simple)

“The oxygen sensor has an internal heater that helps it work during cold starts. That heater has failed or lost power, so the computer is flagging it.”

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