P0140: OBD-II code P0140 means the downstream bank 1 O2 sensor signal is flatlined — the PCM sees no activity, so catalyst efficiency monitoring can't run.

Symptoms

• Check engine light is on; the engine runs and drives normally with no drivability symptoms.
• The catalyst monitor cannot complete, so the vehicle will fail emissions until repaired and the monitor runs clean.
• No power or drivability issues — the fault only affects emissions monitor readiness.

Common causes

• A failed downstream O2 sensor with an internally open element.
• An unplugged or broken sensor wire, or a corroded connector at the sensor.
• A failed sensor heater causing the sensor to stay cold and never activate.

What to do?

Read live downstream O2 voltage — it should sit steady around 0.6-0.8 V once warm. If flatlined at 0.45 V (bias reference), the sensor is open or unplugged. Check wiring; replace the sensor if the wiring tests good.