Monitor Description
WARNING: This page is about a different car, the 2021 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
The ECM monitors the output voltage while the thermostat heater is operating.
The thermostat heater turns on and off according to ON/OFF switching of a transistor inside the ECM.
If a continuous mismatch occurs between the ECM transistor state and the output voltage, the ECM determines there is a malfunction in the thermostat heater circuit and stores a DTC.