P0137: OBD-II code P0137 means the downstream O2 sensor on bank 1 is reading persistently low voltage — usually a bad sensor or a real lean exhaust condition.

Symptoms

• Check engine light is on; the vehicle typically runs and drives normally with no obvious performance issues.
• May accompany a P0420 catalyst efficiency code as the cat fails or reads unusually lean downstream.
• Emissions test failure until the O2 monitor completes clean during a drive cycle.

Common causes

• A failed downstream O2 sensor stuck at low voltage with no switching.
• A real lean exhaust condition (post-cat) from an air leak into the exhaust downstream of the upstream sensor.
• A short-to-ground on the downstream O2 signal wire between sensor and PCM.

What to do?

Read live downstream O2 voltage — should sit around 0.6-0.8 V. If pinned near 0 V, unplug sensor and verify the signal wire is not shorted to ground. Replace the sensor if the wiring is good.