By iCarsoft Team
P0155 Code Fix: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
Code P0155 — O2 Sensor Heater Circuit Malfunction, Bank 2 Sensor 1 — means the engine computer detected a fault in the heater element circuit of your upstream oxygen sensor on Bank 2. That heater brings the sensor up to operating temperature quickly so it can give accurate readings and let the engine enter closed-loop fuel control sooner. When it fails, the sensor warms up slowly, throwing off fuel trim and emissions. The good news is the fix is usually a sensor, some wiring, or a fuse. This guide explains what P0155 means, its causes and symptoms, how to diagnose it, realistic costs, and how to fix and clear it.
If your scan tool returned P0155 — O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1), the ECU has detected a problem with the heating element in the upstream oxygen sensor on Bank 2, the side of the engine that doesn't contain cylinder 1. Modern oxygen sensors have a built-in heater so they reach operating temperature fast after a cold start; when that heater circuit malfunctions, the sensor is slow to respond, and the ECU sets P0155. The usual culprits are a worn sensor, damaged wiring, or a blown fuse.
What Does P0155 Mean?
Your engine relies on an oxygen (O2) sensor in the exhaust to measure leftover oxygen so the ECU can fine-tune the air-fuel mixture. To read accurately, the sensor must be hot, so it has a built-in electric heater that warms it quickly after start-up — letting the engine switch from open-loop to closed-loop fuel control sooner. Bank 2 Sensor 1 is the upstream sensor (before the catalytic converter) on the bank that doesn't include cylinder 1.
P0155 is a generic OBD-II emissions code defined under the SAE J2012 standard. It's set when the ECU sees the heater circuit drawing the wrong current — too high, too low, or none — indicating a short, an open, or excessive resistance. Because the sensor then warms up slowly, fuel trim and emissions suffer until it reaches temperature.
It's closely related to the other O2 heater codes — P0135 (the Bank 1 version) and P0161 (the downstream sensor on the same bank) — and to the signal-side codes on this exact sensor, P0151 (low voltage) and P0152 (high voltage).
Symptoms of P0155
P0155 turns on the Check Engine Light — part of the OBD-II emissions system overseen by the California Air Resources Board. Because a slow-warming sensor throws off fuel control, the symptoms are mostly economy and emissions related. Watch for:
- Check Engine Light — often the only obvious sign.
- Poor fuel economy — fuel trim is off until the sensor warms up.
- Occasional rough idle — from inaccurate mixture adjustments.
- Failed or not-ready emissions test — the O2 monitor can't complete.
- Slow sensor readiness — most noticeable on cold starts and in winter.
Commonly Affected Vehicles
P0155 can occur on any OBD-II vehicle with a heated oxygen sensor and a Bank 2 (V6, V8, or some flat engines). It's commonly reported on Toyota, Ford, Honda, Chevrolet, and Nissan, as well as European brands like BMW, Audi, and Volkswagen — especially on higher-mileage vehicles or in regions with extreme weather, where the heater works hardest.
The Most Common Root Causes (Ranked)
A heater-circuit code points to the sensor's heating element, its wiring, or the power feeding it. Here's the realistic distribution.
| Likelihood | Cause | Why it happens |
|---|---|---|
| ~40% | Faulty O2 sensor heater element | Age, heat exposure, and contaminants degrade the internal heater until it fails. |
| ~25% | Wiring or connector fault | Damaged, corroded, or open wiring in the heater circuit interrupts proper operation. |
| ~20% | Blown O2 heater fuse | A blown fuse cuts power to the heater entirely — a common and cheap-to-fix cause. |
| ~10% | Moisture or corrosion at the connector | Water intrusion on undercarriage connectors corrodes the heater terminals (HT+/HT−). |
| ~5% | ECU fault | Rare. The control module fails to supply or monitor the heater circuit correctly. |
How to Diagnose P0155
Diagnose P0155 from cheapest to most involved: check the fuse, then the heater's resistance and wiring, before replacing the sensor. A scan tool that streams live O2 heater data — like the iCarsoft CR MAX — makes it much faster.
Step 1 — Scan & read live data. Use the iCarsoft CR MAX to confirm P0155 and any companion codes (P0135, P0161, P0151, P0152), and check the heater circuit's voltage and current.
Step 2 — Check the fuse first. A blown O2 heater fuse cuts power to the heater and is a quick, inexpensive find — start here.
Step 3 — Measure the heater resistance. With the connector unplugged, measure the heater element's resistance and compare it to the manufacturer's spec (typically only a few ohms); out-of-range means a bad heater.
Step 4 — Test voltage & wiring. Confirm the heater is getting battery voltage (often with the key on) and inspect the connector terminals (HT+/HT−) for corrosion or water intrusion.
Step 5 — Compare the two banks. If Bank 1's heater works but Bank 2's doesn't, the fault is isolated to Bank 2's sensor or its wiring.
Step 6 — Repair & clear. Fix the confirmed cause, clear the code, and drive a warm-up cycle to confirm P0155 stays off and the monitor completes.
Repair Cost Breakdown
P0155 is usually an affordable fix, and sometimes just a fuse. Figures below reflect typical 2025–2026 US rates.
| Repair | Typical Cost (USD) |
|---|---|
| Professional diagnosis | $75–$150 |
| O2 sensor replacement (part) | $50–$200 |
| O2 sensor labor (1–2 hours) | $50–$150 |
| Wiring / connector repair | $50–$200 |
| O2 heater fuse | Under $20 |
Watch out: left unresolved, P0155 keeps emissions monitors from completing and can let poor fuel trim strain the catalytic converter over time — a far costlier repair.
Diagnose P0155 with the iCarsoft CR MAX
A basic reader confirms P0155 but can't show you whether the heater is shorted, open, or just unpowered. The iCarsoft CR MAX streams live O2 heater voltage and current and lets you compare both banks, so you can tell a fuse or wiring fault from a failed sensor before buying parts.
- Live O2 heater data — watch heater voltage and current in real time.
- Bank-by-bank comparison — isolate the Bank 2 sensor or wiring fast.
- Read companion codes (P0135, P0161, P0151, P0152) for the full O2 picture.
- Broad multi-brand coverage for domestic, Asian, and European vehicles.
- Clear codes and confirm the monitor completes after the repair.
Preventive Maintenance — Keep P0155 Away
- Follow the maintenance schedule — routine service reduces wear on sensors and wiring.
- Use quality replacement O2 sensors — they last longer and read more reliably.
- Protect the heater-circuit wiring — shield it from heat and moisture during repairs.
- Address O2 codes early — before emissions monitors won't pass at inspection time.
Frequently Asked Questions
Can I drive with a P0155 code?
Short-term, yes — it doesn't usually affect drivability much. But long-term it's not advisable: you may see poor fuel economy, fail an emissions test, or strain the catalytic converter.
What does "Bank 2, Sensor 1" mean?
Bank 2 is the side of the engine that doesn't contain cylinder 1. Sensor 1 is the upstream sensor, mounted before the catalytic converter.
Is P0155 always a bad oxygen sensor?
No. It's often a blown O2 heater fuse or damaged wiring. Check the fuse and measure the heater resistance before replacing the sensor.
What's the difference between P0155 and P0135?
They're the same heater-circuit fault, but P0155 is on Bank 2 Sensor 1 while P0135 is on Bank 1 Sensor 1.
Why is P0155 more noticeable in winter?
The heater matters most on cold starts, so a weak or failing heater is more likely to set the code in cold weather.
Bottom Line
P0155 means the heater circuit in your Bank 2 upstream oxygen sensor has a fault, so the sensor warms up slowly and throws off fuel trim and emissions. Because it's often something cheap — a blown fuse or damaged wiring — start there: check the fuse, measure the heater's resistance, and compare both banks with a capable tool like the CR MAX before replacing the sensor. Fix the real cause, clear the code, and run a warm-up cycle so the monitor completes — and don't let it linger, since it can keep you from passing inspection and wear on the converter over time.
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