P0155moderate severityGenerally driveableP0155 Code: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
Written in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026
What it means in plain English
Oxygen sensors contain a small electric heater that brings them up to operating temperature, around 600°F, within seconds of startup — critical for the upstream sensor, since the ECM relies on it to fine-tune the fuel mixture in real time. Bank 2 Sensor 1 is the upstream sensor, positioned before the catalytic converter, on the engine bank that does not contain cylinder #1. P0155 means the ECM checked this heater circuit's electrical current and found it outside the expected range. Because this is an upstream sensor that directly influences fuel control, a heater fault here matters more for fuel economy and emissions than the same fault on a downstream sensor would.
Most likely causes — ranked
#1 Blown fuse for the O2 sensor heater circuit
most likelyFix: Check the fuse box for a fuse labeled O2 sensor, sensor heater, or EFI — some vehicles split heater circuits across multiple fuses. This is the fastest, cheapest check and should always be done first.
#2 Damaged or corroded wiring/connector at the Bank 2 Sensor 1 O2 sensor
likelyFix: This upstream sensor sits close to the hot exhaust manifold, where its wiring is prone to heat damage, chafing, and connector corrosion over time. Inspect the harness closely for melted insulation or breaks, and check the connector pins for corrosion. Repair any damage found before condemning the sensor.
#3 Burned-out heater element inside the O2 sensor
likelyFix: Unplug the connector and use a multimeter to check resistance across the heater circuit pins — most sensors read roughly 3 to 30 ohms when healthy (check your factory spec); an open circuit means the heater has failed. Replace the sensor with an OEM-equivalent part for reliable results.
#4 Faulty PCM heater control circuit (rare)
possibleFix: Only after confirming the fuse, wiring, and sensor heater resistance are all good should the PCM's internal driver circuit be suspected. A shop with factory diagnostic software can confirm this before recommending PCM-level work.
Check engine light on? Get a free code scan
If your check engine light is on, most auto-parts stores (AutoZone, O'Reilly, Advance) will read the code for FREE — no purchase required, just walk in. Bring the code back here and look it up at eli5cars.com/obd2 for the plain-English explanation.
Note: a scan only helps when there's an active fault code. For purely mechanical symptoms (noises, vibrations, leaks), a scan may show nothing — the suspects listed above are your starting point.
Want to read the codes yourself?
First, the free option: AutoZone, O’Reilly, and Advance Auto Parts read your codes for free — no purchase needed. That’s the cheapest way to get the code to look up here. If you’d rather have your own so you can check anytime (and clear a code after a fix), the Veepeak OBDCheck BLE is an inexpensive, reliable plug-in reader that pairs with a free phone app and works on any car sold since 1996.
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Frequently asked questions
Can I drive with P0155?
Yes, but expect longer cold-start warm-up times before the ECM enters closed-loop fuel control, along with slightly worse cold-start fuel economy and emissions. It's not an urgent safety issue but shouldn't be ignored for months.
Is P0155 the same as a bad O2 sensor?
Not necessarily — it's specific to the heater circuit. A blown fuse or damaged wire can trigger this code on a perfectly good sensor, so always check those first since they cost little to nothing to rule out.
Which sensor is Bank 2 Sensor 1?
Sensor 1 means upstream, positioned before the catalytic converter, and Bank 2 is the side of the engine that does not contain cylinder #1, which is relevant on V6/V8 engines. Check a service diagram for your specific vehicle to confirm which physical side that is.
Why does the upstream sensor's heater matter more than the downstream one?
The upstream sensor feeds the ECM's real-time fuel mixture calculations, so a slow-to-heat or non-functioning heater delays accurate closed-loop fuel control after every cold start. The downstream sensor mainly monitors catalytic converter efficiency, so its heater has less impact on how the engine actually runs.