Upstream O2 Sensor Stays at Center

Name of Code:
Upstream O2 Sensor Stays at Center

When Monitored:
With the engine running for more than 2 minutes and engine temperature greater than 17O°F.

Set Condition:
The upstream oxygen sensor signal voltage is between 0.35 volts and 0.55 volts for 1.5 minutes.

Theory of Operation:
The upstream 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 upstream oxygen sensor tells the PCM how well its output signals are controlling the air/fuel ratio. Variations in the signal from this 02 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:
- Sensor output wire open
- O2 sensor failure
- PCM failure
- Dirt/moisture/grease causing voltage tracking in connector
- Connector wires