Blower motor switch has failed. The blower motor switch is a component that controls the speed of the blower motor. When the switch fails, it can cause the blower motor to only operate at high speed.
Wiring issue. A wiring issue can also cause the blower motor to only operate at high speed. A loose or damaged wire can prevent the blower motor from receiving the correct voltage, which can cause it to only operate at high speed.
Blower motor has failed. If the blower motor has failed, it will not be able to operate at any speed. This is a more serious issue that will require the blower motor to be replaced.
