Circuit Description

Heated oxygen sensors (HO2S) are used for fuel control and post catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the vehicle is first started the powertrain control module (PCM) operates in an Open Loop mode, ignoring the HO2S signal voltage when calculating the air-to-fuel ratio. The PCM supplies the HO2S with a reference, or bias, voltage of about 450 mV. The HO2S generates a voltage within a range of 0-1,000 mV that fluctuates above and below bias voltage once in Closed Loop. A high HO2S voltage output indicates a rich fuel mixture. A low HO2S voltage output indicates a lean mixture. Heating elements inside the HO2S minimize the time required for the sensors to reach operating temperature, and to provide an accurate voltage signal. The HO2S 2 is used for post catalyst monitoring. If the PCM detects that the HO2S 2 voltage is above a calibrated amount for an excessive amount of time, DTC P0138 will set.

The HO2S 2 has the following circuits:

• A HO2S 2 high signal circuit
• A HO2S 2 low signal circuit
• A HO2S 2 heater ignition voltage circuit
• A HO2S 2 heater ground circuit