Heated Oxygen Sensor (HO2S)

There are 4 HO2S located in the exhaust system. One in front of each catalytic converter and one after. Each HO2S consists of a titanium metal sensor surrounded by a gas permeable ceramic coating. Oxygen in exhaust gas defuses through ceramic coating and reacts with the titanium wire, altering resistance of wire. ECM monitors front HO2S resistance changes and calculates amount of oxygen in exhaust gas, which indicates a rich or lean mixture. ECM adjusts on-time of each injector to maintain correct air/fuel ratio. ECM monitors 2 rear HO2S to determine catalytic converter operating efficiency.

All HO2S are electrically heated to ensure they achieve operating temperature as quickly as possible after start-up. Both front and both rear sensor heaters are connected in parallel. If front HO2S wiring is crossed, vehicle will operate properly until sensors reach operating temperature. ECM will then cause one bank of cylinders to run very rich and the other bank to run very lean. This will cause engine to misfire, idle rough and emit black smoke, with possible catalytic converter damage.

If there is a HO2S failure, system will default to an open-loop operation and ECM will base fuel requirements on other sensor inputs. A fault is indicated by illumination of the MIL.