Circuit Description

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in exhaust gas compared to outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts injection pulse duration to achieve ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

Under the condition in which the front HO2S signal is not input, the ECM circuits will read about .3 volt continuously. Therefore, for this diagnosis, the time that output voltage is 200-400 millivolts is monitored, and the diagnosis checks that this time is not inordinately long.