Summary
P0155 means the heater circuit in the upstream oxygen sensor on Bank 2 has failed. This is the Bank 2 equivalent of P0135. The most common cause is a burned-out heater element (60%), followed by a blown fuse or relay (15%) and wiring issues (15%). If both P0135 and P0155 are present, check the shared fuse first — one blown fuse can kill heaters on both banks. Fix is usually the sensor: $25–$120 part, 20–30 min.
Severity: Low to Moderate — Bank 2 fuel control runs open-loop until sensor warms up
Safe to drive: Yes
Repair cost: $25–$200 depending on cause
DIY difficulty: Moderate
What does P0155 mean?
The Bank 2 upstream O2 sensor controls fuel delivery for that bank. Its internal heater brings it to operating temperature (~600°F) within seconds of engine start. P0155 sets when the PCM detects a fault in this heater circuit — typically an open element (no current draw) or a short.
Without the heater, the sensor relies on exhaust heat to warm up, which takes several minutes. During that time, the PCM runs Bank 2 in open-loop mode with pre-programmed fuel maps. Bank 2 fuel economy and emissions suffer during warm-up.
On V-configuration engines, Bank 2 is the side without cylinder 1. On inline engines with a single bank, you won't see P0155.