Thermostat: The thermostat is responsible for regulating the temperature of the engine coolant and sending a signal to the heater control module when it needs to turn on or off. If the thermostat is faulty, it may be stuck in the open position, causing the heater to run constantly.
Heater control module: The heater control module receives the signal from the thermostat and sends a signal to the heater fan and water valve to turn them on or off as needed. If the heater control module is faulty, it may send a constant signal to the heater fan and water valve, causing the heater to run constantly.
Heater fan: The heater fan is responsible for blowing warm air into the passenger compartment. If the heater fan is faulty, it may not be able to blow air at all or may only be able to blow air at a low speed. This can cause the heater to feel like it is only working on full, even if the thermostat and heater control module are working properly.
Water valve: The water valve is responsible for controlling the flow of coolant to the heater core. If the water valve is faulty, it may not be able to open fully, causing the heater to feel like it is only working on full, even if the thermostat, heater control module, and heater fan are all working properly.
If you have checked all of these things and the heater still only works on full, you may need to take your car to a mechanic for further diagnosis and repair.
