Downstream O2 Sensor Shorted to Ground

Name of Code:
Downstream O2S Voltage Shorted to Ground

When Monitored:
With the ignition key off, after the oxygen sensor cools down; and after the next key on provided that it is a cold start (coolant temperature less than 100.4°F).

Set Condition:
The downstream oxygen sensor signal voltage is below 0.156 volt prior to O2 sensor heater test (cool down period), and O2 voltage less than 0.156 volt after the key on in the subsequent start.

Theory of Operation:
The downstream oxygen sensor is a voltage generating device. The PCM receives exhaust gas information from this O2 sensor. The sensor detects exhaust gas content by a galvanic reaction within the sensor that produces a voltage after measuring the amount of oxygen in the exhaust gases, the downstream oxygen sensor tells the PCM how well the catalytic converter is oxidizing HO and CO components and reducing NOx components in the engine exhaust gas. Variations in the signals from this O2 sensor serve as an indicator of oxygen content. The catalytic converter absorbs the oxygen in the exhaust gas when the fuel/air mixture is lean. The downstream O2 sensor indicates a deficiency of oxygen when the upstream O2 sensor detects an abundance of oxygen and vice versa.

Possible Causes:
- Sensor output wire shorted to another circuit
- Dirt/moisture/grease causing voltage tracking in connector
- O2 sensor failure
- Powertrain control module failure
- Connector terminals/wires