Sunfire L4-138 2.3L DOHC VIN D MFI (1995)
Oxygen Sensor: Description and Operation
Catalyst Monitor Oxygen Sensors
Rear HO2S
DESCRIPTION
The Rear Heated Oxygen Sensor (HO2S 2) is used on California vehicles only and is used to monitor the oxygen storage capability of the catalytic
converter. Similar to the O2S, the HO2S 2 reacts with the oxygen content in the exhaust stream to produce a voltage signal only after the catalytic
converter. This voltage ranges from approximately 0.1 volt (high oxygen - lean mixture) to 0.9 volt (low oxygen - rich mixture). Unlike the 02S,
the HO2S 2 is normal when it's activity appears lazy or inactive. The PCM compares readings from both the front 025 and the H025 2 to
determine the catalyst efficiency and will illuminate the MIL if this efficiency falls below a calibrated level.
