The blower motor resistor is a crucial component that controls the different fan speeds. It's essentially a variable resistor that regulates the voltage going to the blower motor. When it fails, typically the motor will only run at its highest speed (or not at all). Lower speeds are controlled by reducing the voltage through the resistor, and if that resistor is faulty, those lower speed circuits are broken.
Other less likely possibilities include:
* Blower motor itself: A failing blower motor might only work at full speed before completely dying.
* Wiring issues: A short or break in the wiring leading to the resistor or blower motor could cause intermittent operation.
* Fuse or circuit breaker: Though less likely given the symptom, a blown fuse specifically for the blower motor circuit could cause the problem, but usually would completely prevent operation even at high speed.
In short: The most probable cause is a bad blower motor resistor. It's a relatively inexpensive and easy-to-replace part, making it a good place to start diagnosing the problem. You can find replacement parts at auto parts stores or online. A mechanic can also diagnose and fix the issue.
