P0155: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
TL;DR
P0155 = O2 sensor heater circuit (Bank 2, Sensor 1). Severity: low. Top causes: failed O2 sensor heater (55%), blown fuse/wiring (25%), connector fault (15%), ground (5%). Bank 2 = the side opposite cylinder #1.
Can I keep driving with P0155?
IF it’s just P0155 → safe to drive; the main effect is slower closed-loop entry and slightly worse economy. Fix it to pass emissions and restore efficiency.
Symptoms
- Check engine light on
- Usually no major drivability change
- Slightly worse fuel economy
- May fail emissions/inspection
- Slower warm-up to closed loop
Top causes (ranked by probability)
| Likely cause | Probability | Notes |
|---|---|---|
| Failed O2 sensor heater element | 55% | Most common — replace the sensor |
| Blown heater fuse or wiring fault | 25% | |
| Corroded or loose connector | 15% | |
| Ground fault | 5% |
What does P0155 mean?
Technical explanation
The heater brings the upstream O2 sensor to operating temperature quickly so the ECM can enter closed-loop fuel control sooner. P0155 sets when the heater circuit on Bank 2, Sensor 1 is out of range. Causes include an open heater element inside the sensor, a blown heater fuse, an open/short in the wiring, a poor connector, or a ground fault. It mainly delays closed-loop operation, increasing emissions and slightly worsening economy.
In simple terms
The front oxygen sensor on one side of the engine has a built-in heater so it works sooner after startup. P0155 means that heater on Bank 2 isn’t working — usually the sensor itself or a fuse/wire. The car runs, but it takes longer to optimize fueling, so economy and emissions suffer a bit.
How to diagnose P0155 (step by step)
- Confirm the bank/sensor. Bank 2 is the cylinder bank that does NOT contain cylinder #1; Sensor 1 is upstream.
- Check the heater fuse. A blown fuse can disable the heater circuit.
- Test the heater element resistance. An open element confirms a failed sensor.
- Inspect wiring and connector. Look for opens/shorts and corrosion.
- Replace the O2 sensor if the heater is open. The heater is internal — replace the sensor.
Repair options & cost
- Replace the Bank 2 upstream O2 sensor
- Replace the heater fuse / repair wiring
- Repair the connector
- Fix the ground
🔧 Doing it yourself? Buy the part: Oxygen (O2) sensor
| DIY cost | $30–$200 |
|---|---|
| Workshop cost | $150–$400 |
| Repair time | 30–60 minutes |
Costs are local ballpark ranges and vary by region and vehicle.
Tools you’ll need
Scan your car: recommended OBD-II scanners →Vehicle-specific notes
- Bank 2 is the side of the engine WITHOUT cylinder #1 — confirm before buying a sensor.
- The heater is built into the O2 sensor, so an open heater means replacing the sensor.
- P0155 mirrors P0135 (Bank 1, Sensor 1 heater).
Common mistakes to avoid
- Replacing the Bank 1 sensor by mistake (wrong bank)
- Overlooking a blown heater fuse
- Ignoring a corroded connector
- Using a low-quality sensor that fails again
Frequently asked questions
What does P0155 mean?
The heater circuit in the upstream oxygen sensor on Bank 2 isn’t working — usually a failed sensor heater, a blown fuse, or wiring problems.
Which side is Bank 2?
Bank 2 is the cylinder bank that does NOT contain cylinder #1. Confirm your engine’s layout before buying a sensor so you replace the correct one.
Is P0155 safe to drive with?
Yes. The main effect is slower closed-loop entry and slightly worse economy. Fix it to pass emissions and restore efficiency.
P0155 summary
| Meaning | O2 heater fault (Bank 2, Sensor 1) |
|---|---|
| Severity | Low |
| Safe to drive? | Yes — emissions related |
| Top cause | Failed O2 heater (55%) |
| DIY cost | $30–$200 |
| Shop cost | $150–$400 |