P0155: The upstream oxygen sensor heater on the passenger side is not working
O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)
The heater in the upstream oxygen sensor has a fault (bank 2). Get it checked within a week or two. If the engine runs rough or the light flashes, stop driving.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
What it means
P0155 is the Bank 2 version of P0135. Bank 2 is the side of a V6 or V8 engine that does not contain cylinder 1, usually the passenger side. The upstream sensor heater is supposed to warm the sensor to 600 degrees or more within about 30 seconds of a cold start. Without that heat, the sensor takes several minutes to activate, and the engine runs on a default fuel map the entire time. On V-engines, both banks have their own upstream sensors, and a fault on Bank 2 can go unnoticed because the engine may still run fine on Bank 1 data.
Common causes
- Failed heater element in the Bank 2 upstream O2 sensorHeat cycling over many miles burns out the internal heater wire. Common past 80,000 miles.
- Blown heater circuit fuseCheck the fuse box for the O2 heater circuit before buying a sensor.
- Wiring or connector damage on the Bank 2 sideThe Bank 2 sensor harness routes differently. Heat and chafing from the exhaust manifold are common damage sources.
What the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- Fuse replacement
- Wiring repair or connector
- Upstream O2 sensor (Bank 2)
Symptoms
- Check engine light on, steady
- Slightly richer running and worse fuel economy on short trips
- Possibly rough-ish idle for a minute after cold start
- Emissions test failure
What to do next
This code only appears on V6, V8, or flat-six engines. If you have a four-cylinder, confirm which bank is Bank 1 for your specific engine before ordering parts. Check the fuse and wiring first. Bank 2 sensors can be harder to access on some engines and may require removing engine covers or exhaust heat shields, which can add labor cost.
Lewis reads P0155 alongside any related oxygen sensor codes and shows your live Bank 2 fuel trims, so you can see how the engine is running on that side. Get Lewis on the App Store or Google Play.
Related codes
FAQ
- How do I know which side is Bank 2?
- Bank 1 is the side with cylinder 1. Bank 2 is the other side. Cylinder numbering varies by make and model. A quick search for your specific engine will confirm which side is which.
- P0155 on a four-cylinder engine -- is that possible?
- It should not be. Four-cylinder engines have only one bank. If you see P0155 on a four-cylinder, that likely points to a scan tool or ECU data issue.
- Will P0155 damage the engine?
- Not directly. The main risk is slightly worse fuel economy and increased cold-start emissions until the sensor is warm.
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