Monitor Description

The ECM uses the front HO2 sensor to optimize the air-fuel ratio with closed-loop fuel control. This control helps to decrease exhaust emissions by keeping the conditions optimum for the TWC to work at maximum efficiency. The front HO2 sensor detects the oxygen level in the exhaust gas, and provides the ECM with feedback to allow it to control the air-fuel ratio accurately.

The front HO2 sensor voltage output ranges from 0 V to 1 V. When the sensor signal voltage is less than 0.45 V, the air-fuel ratio is lean. Conversely, when it is more than 0.45 V, the air-fuel ratio is rich. If the front HO2 sensor does not indicate rich at all despite the closed-loop fuel control conditions being met for a certain period of time, the ECM determines that closed-loop fuel control is malfunctioning. The ECM illuminates the MIL and sets the DTC.