Upstream Heated and Downstream Heated Oxygen Sensors

Upstream Heated and Downstream Heated Oxygen Sensors
The HO2S provide the Powertrain Control Module (PCM) with voltage and frequency information related to the oxygen content of the exhaust gas. (Refer to the PCM Inputs for a description of how the HO2S operates.)

In addition to providing the PCM with indications of how rich/lean the engine is operating, the upstream HO2S signal serves as an input to the HO2S monitor. The downstream HO2S signal is an input to the Catalyst Efficiency monitor. (Refer to the OBD II Monitors for specific information on these monitors.)