If a blower motor only works on high speed, what is the most likely cause?

When a blower motor operates only on high speed, the most likely cause is a defective blower resistor. The blower resistor is responsible for controlling the speed of the blower motor in conjunction with the blower switch. In systems with multiple speed settings, the resistor decreases voltage to the blower motor, allowing it to operate at lower speeds. If the resistor fails, it may either allow full voltage to bypass all lower speed settings or fail entirely, resulting in the motor running only at high speed.

In this scenario, the other components mentioned do not primarily influence the ability of the motor to operate at different speeds. The blower relay typically controls the on/off function of the motor rather than speed variations, the blower switch allows the user to select different speeds but would not cause a complete failure to operate at lower speeds if functioning correctly, and a blown fuse or tripped circuit breaker would result in no operation at all, rather than restricting operation to just high speed.