O2S Stays Above Center (Rich)

O2S STAYS ABOVE CENTER (RICH)

When Monitored
With the engine running in closed loop and the coolant temperature above 170°F.

Set Conditions
The oxygen sensor signal voltage stays above 0.5 volts but less than 1.2 volts without toggling for at least 8 minutes.

Theory of Operation
The oxygen sensor is a voltage generating device. The Powertrain Control Module (PCM) receives exhaust gas information from the 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 oxygen sensor tells the PCM how well its output signal is controlling the air/fuel ratio. Variations in the signals from the O2 sensor serve as air/fuel ratio indicators. Changes in the sensor signal occur because the air/fuel ratio is constantly changing. When oxygen content is low (rich mixture), the voltage signal will be approximately 1 volt. When oxygen content is high (lean mixture), the voltage signal will be low, approximately 0.1 volt.

Possible Causes
- High fuel pressure.
- Other engine sensor calibrations.
- Ignition system failure.
- O2 sensor failure.
- Mechanical engine problem.