If the blower motor only operates in the number 4 position, what component could be failing?

When the blower motor operates only at the highest speed setting (number 4), it typically indicates a problem with the blower motor resistor. The blower motor resistor is responsible for regulating the speed of the blower motor by changing the resistance and controlling the current flowing to the motor.

In many vehicles, the lowest three speeds (1, 2, and 3) use the resistor to reduce voltage and, consequently, the speed of the blower motor. When the resistor fails, the blower motor loses its ability to operate at these lower speeds and may only function at full speed. This happens because, in the full-speed position, power is sent directly to the blower motor without passing through the resistor, which is why it continues to operate in that mode.

In contrast, while the blower motor itself could be checked for issues, its functionality at the highest setting indicates that the motor is likely fine. The blower motor ground would also not be a direct cause since a ground issue would typically prevent the motor from running at all. The blower motor relay controls the power to the motor but tends to be either fully operational or not, rather than presenting a partial failure as seen here. Thus, the blower motor resistor is the key component to consider when the motor only works