Pre-Catalyst (Front)

The wide range Heated Oxygen Sensor (HO2S) calculates the amount of oxygen in the exhaust stream more accurately than the toggling style HO2S. The wide range HO2S sensor acts more like an air fuel sensor. The Engine Control Module (ECM) supplies a voltage (approx. 28 volt) to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. When the system is lean, the oxygen level will be high and the reference voltage also will be high. When the oxygen level is low, the reference voltage will also be low. The ECM monitors the variation in voltage and attempts to keep the voltage constant by increasing or decreasing the amount of current flow to the HO2S. Using this information allows the ECM to maintain the proper air/fuel ratio.