P0155: OBD-II code P0155 flags a fault in the upstream O2 sensor heater circuit on bank 2 — the sensor can't reach operating temperature fast enough for closed-loop control.

Symptoms

• Check engine light is on; the engine may run rich on bank 2 longer than usual after a cold start.
• Poor fuel economy at start-up as the PCM stays in open-loop enrichment on bank 2 waiting for a valid O2 signal.
• The vehicle will fail an OBD-II emissions test until the O2 heater monitor completes for bank 2.

Common causes

• A failed bank 2 upstream O2 sensor heater element (open circuit inside the sensor) — the most common cause.
• A blown fuse feeding the O2 sensor heater circuit, or a corroded ground on the heater side.
• Damaged wiring or a corroded connector at the sensor, common near the exhaust manifold on bank 2.

What to do?

Measure heater resistance across the sensor's two heater pins with a DMM — should read 8-15 Ω. Infinite means open heater — replace the sensor. Check fuse and verify battery voltage at the heater feed pin with ignition on.