Here's why this happens:
* Resistor Pack Failure: The most common reason. The resistor pack, usually located somewhere near the blower motor (often under the dash), is made up of resistors that burn out over time. When one or more fail, the circuit is broken, leaving only the high speed setting functional.
* Wiring Issues: A short circuit or a broken wire in the wiring harness leading to the resistor pack could also prevent the lower speeds from working, forcing the fan to run on high.
* Blower Motor Issues (Less Likely): While less probable than the resistor pack, a problem with the blower motor itself *could* contribute if the motor is only functioning properly at full power. This is less likely, however, as a failing motor usually leads to inconsistent operation at any speed, or no operation at all.
* Switch Problems (Less Likely): A faulty blower motor speed switch could theoretically contribute, but is less likely to cause *only* the high speed to work. Typically, this would cause multiple speeds to fail or operate intermittently.
In short: The resistor pack is the most likely culprit. It's relatively inexpensive and easily replaced, making it the first thing to check and replace if you're handy. If you're not comfortable working on your vehicle's electrical system, take it to a mechanic.
