P0131: OBD-II code P0131 means the upstream oxygen sensor on bank 1 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, hesitate, or misfire under load.
• Fuel economy may improve slightly because the mixture is lean, but power drops and the engine hesitates.
• The engine may buck or stumble on acceleration as the PCM tries to compensate for the persistent lean signal.

Common causes

• A failed upstream O2 sensor stuck at low voltage (0.1-0.2 V), unable to switch.
• A real lean condition from a vacuum or intake leak upstream of the sensor.
• Low fuel pressure, a clogged fuel filter, or a weak fuel pump reducing the fuel supply.

What to do?

Read live O2 sensor voltage at idle and light throttle — it should switch between about 0.1 V and 0.9 V once warm. If stuck low, check for vacuum leaks and low fuel pressure first. Replace the O2 sensor if it doesn't switch after roughly two minutes of warm running.