P0151: OBD-II code P0151 means the upstream oxygen sensor on bank 2 is reading persistently low voltage — usually a real lean condition or a dead sensor.

Symptoms

• Check engine light is on; the engine may run lean on bank 2, hesitate, or misfire under load.
• Fuel economy may improve slightly on that bank but power drops and the engine stumbles on acceleration.
• The engine may buck or hunt as the PCM tries to compensate for the persistent lean signal from bank 2.

Common causes

• A failed upstream bank 2 O2 sensor stuck at low voltage (0.1-0.2 V), unable to switch.
• A real lean condition on bank 2 from a vacuum or intake leak upstream of the sensor.
• Low fuel pressure or a partial fuel supply issue affecting bank 2 more than bank 1.

What to do?

Read live bank 2 upstream O2 voltage — it should switch between about 0.1 V and 0.9 V once warm. If stuck low, check for vacuum leaks on that bank and verify fuel pressure. Replace the O2 sensor if it doesn't switch after warm running and no air/fuel-side cause is found.