Circuit Description

Using Heated Oxygen Sensors (HO2S) for fuel control decreases amount of time required to enter closed loop operation. Heated oxygen sensors when used post-catalyst, ensure sufficient heat is maintained for accurate catalyst monitoring.

When HO2S is cold, signal voltage at ECM is about 450 mV. When ignition is turned to on position, switched battery voltage is supplied to HO2S heaters via main relay. ECM controls heater operation by first modulating ground circuit when sensors are cold, then turning them on continuously after predetermined amount of time. This prevents possibility of thermal shock to sensors by controlling sensor's rate of heating. As heater reaches operating temperature, HO2S voltage responds by changing from bias voltage range to normal operation.

On cold start, ECM will track how much time it takes for HO2S voltage to go above or below bias voltage range. If ECM determines that it took too much time for HO2S to enter into normal operating range, this DTC will set. The time it takes HO2S to reach operating temperature is based on amount of airflow into engine and start-up coolant temperature.

Conditions required to set DTC are:

• ECM calculated HO2S heater resistance greater than 9.56 ohms or less than 2.45 ohms.