Oxygen Sensor: Description and Operation

PURPOSE
The oxygen sensor tells the PCM how well its output signal is controlling the air/fuel ratio.

OPERATION
The oxygen sensor is a voltage generating device. The PCM receives exhaust gas information from the O2 sensor. The sensor detects exhaust gas content by a galvanic reation within the sensor that produces a voltage.

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 the 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.

NAME OF CODE: O2S Stays Above Center (Rich)

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

Set Condition:
The oxygen sensor signal voltage stays between 0.5 and 1.2 volts without toggling for at least 8 minutes.

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

NAME OF CODE: O2S Stays Below Center (Lean)

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

Set Condition:
The oxygen sensor signal voltage stays below 0.5 without toggling for at least 8 minutes.

Possible Causes:
- Low fuel pressure
- Vacuum Leak
- Other engine sensor calibrations
- Ignition system failure
- O2 sensor failure
- Mechanical engine problem

NAME OF CODE: Upstream O2S Shorted To Voltage

When Monitored:
With the ignition key ON.

Set Condition:
The upstream oxygen sensor signal voltage is above 1.2 volts for 3 seconds.

Possible Causes:
- Sensor output wire shorted to another circuit
- Dirty/wet connection causing voltage tracking in connector
- O2 sensor failure
- PCM failure
- Connector terminals
- Connector wires

NAME OF CODE: Upstream O2S Stays at Center

When Monitored:
With the engine running for more than 2 minutes and engine temperature greater than 176° F.

Set Condition:
The upstream oxygen sensor signal voltage is 0.5 volts for 1 minute.

Possible Causes:
- Sensor output wire open
- O2 sensor failure
- PCM failure
- Connector terminals
- Connector wires