Oxygen Sensor Heater Control
WARNING: This page is about a different car, the 1997 Lexus SC 300. However, it is still accessible from the selected car via links, so may be relevant.
The ECM controls the operation of the oxygen sensor heater according to intake air volume and engine speed. If engine load is light and exhaust gas temperature is low, the heater is turned on to maintain sensor efficiency. When engine load is high and exhaust temperature is high, the heater is turned off to prevent deterioration of the sensor.