Downstream O2 Sensor Stays at Center

Name of Code:
Downstream O2 Sensor Stays At Center

When Monitored:
With the vehicle engine running for more than two minutes, engine temperature greater than 170°F, in closed loop.

Set Condition:
If oxygen sensor voltage stays between 0.29 volts and 0.72 volts for 30 seconds, the PCM pulls the sensor voltage to 5.0 volts for 60 seconds and monitors the sensor voltage. If the voltage stays at 5.0 volts, this trouble code will set, indicating no activities.

Theory of Operation:
Effective control of exhaust emissions is achieved by an oxygen feedback system. The downstream oxygen sensor (O2S), located in the exhaust path, monitors the exhaust emissions. Once the sensor reaches its normal operating temperature of 300°-350° C (572°-662° F), it generates a voltage inversely proportional to the amount of oxygen in the exhaust. This voltage is used to calculate the efficiency of the catalytic converter. The downstream oxygen sensor is monitored for proper response to assure a correct catalytic converter efficiency calculation. A good sensor will indicate a rich condition when at WOT (due to the enriched F/A ratio), and a lean condition when in decel fuel shut-off (due to the lean F/A ratio).

Possible Causes:
- Sensor output wire open
- Dirt/moisture/grease inside connector
- Open ground circuit
- Oxygen sensor failure
- PCM failure