Heated Oxygen Sensors
WARNING: This page is about a different car, the 2002 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.
Heated Oxygen Sensors (HO2S) monitor oxygen content of exhaust gases. HO2S are mounted before and after catalytic converter in exhaust stream. When HO2S is at operating temperature, a voltage signal is produced, which varies according to oxygen content of exhaust gases. HO2S uses a built-in heating circuit. The heating circuit is used to bring the HO2S up to operating temperature, enabling faster conversion to closed-loop operation. Front HO2S monitors fuel mixture and is used by PCM to constantly adjust fuel mixture rich or lean. Rear HO2S is used to monitor catalyst efficiency. Signals between front and rear HO2S are compared and if sufficient difference is not produced, a DTC will set and MIL will illuminate.