OBD II Readiness Monitors

Non-Continuous Monitors

Oxygen (O2) Sensor

The oxygen sensor monitor allows the PCM to verify that the O2 sensors are properly calibrated and functioning without noticeable deterioration under normal engine operating conditions. This ensures that when the PCM is controlling fuel mixture during closed-loop operation, an actual air/fuel ratio of 14.7:1 is being combusted in the cylinders. When the air/fuel ratio moves away from this ideal point catalytic converter efficiency decreases and emissions increase.

The PCM enables the oxygen sensor monitor after the engine and catalytic converter have been brought up to normal operating temperature, to reach closed loop operation, and the vehicle is maintaining a steady speed above 25 mph for several minutes. For the monitor to run, the check engine light needs to be commanded off by the PCM and the continuous monitors must have completed successfully.

The PCM tests all the oxygen sensors when the monitor is enabled. The upstream O2 sensors are checked for proper voltage threshold and response rate to air/fuel mixture changes (switching from rich to lean). The downstream O2 sensors are checked for proper voltage threshold and the ability to recognize lean conditions. Both sensors are monitored for signal rationality.

While the monitor is running, the PCM executes a fixed frequency fuel control routine and observes the upstream sensors’ output voltage and switching response frequency. The PCM also shuts off the fuel injectors during a period of deceleration and observes the downstream sensors’ voltage response to the lean condition.

The check engine light is illuminated by the PCM when an oxygen sensor fault has been detected after 2 drive cycles.