What component needs to be replaced if a vehicle's blower motor only works on high?

When a vehicle's blower motor only operates on the high setting, it indicates that the lower speed settings (like low and medium) are not functioning. The component responsible for regulating the speed of the blower motor is the blower resistor.

The blower resistor is designed to allow varying amounts of electrical resistance in the circuit, which in turn controls the speed of the blower motor. When the blower motor is only functional at high speed, it typically means that the resistor has failed or short-circuited. In high-speed operations, the blower motor receives full voltage directly, bypassing the resistor, which usually allows it to operate at lower speeds. Thus, the resistor is the component that needs to be replaced to restore full functionality to all blower motor speed settings.

The other components mentioned, such as the blower motor itself, relay switch, and thermostatic switch, do not directly control the variable speeds of the blower motor in the same way the resistor does.