The resistor pack is a component that reduces the voltage to the blower motor for lower speeds. When it fails, the lower speeds stop working because the appropriate voltage isn't being supplied, while the higher speeds (which often bypass the resistor pack entirely or use a different portion of it) continue to function.
Other, less likely possibilities include:
* Worn blower motor brushes: While less common than a resistor pack issue for this specific symptom, worn brushes can cause the motor to struggle at lower speeds where it needs more torque. It would likely affect all speeds to some extent, though.
* Wiring issues: A break or short in the wiring leading to the blower motor or the resistor pack could cause intermittent problems or issues with specific speeds. This is less likely to cause *only* speeds 1-3 to fail.
* Blower motor itself: A failing blower motor *could* exhibit this symptom, but it's less probable than a resistor pack problem given the specific nature of the fault.
In short: Start by checking the resistor pack. It's usually relatively accessible and inexpensive to replace. If replacing the resistor pack doesn't solve the problem, then further diagnosis of the wiring and blower motor might be necessary.
