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.

Codes › P0155

P0155: The upstream oxygen sensor heater on the passenger side is not working

O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)

Moderate severityCan you keep driving?

Yes, you can drive for now. Fuel economy and cold-start emissions will suffer, but the engine runs. Fix it within a week or two to avoid fuel trim issues during warm-up.

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 it costs

Fuse replacement$5 to $20
Wiring repair or connector$80 to $200
Upstream O2 sensor (Bank 2)$150 to $375 installed

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 shows the Bank 2 sensor warm-up trace alongside fuel trim data, so you can see how long the engine is running open-loop on cold starts. Download Lewis.

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.