The blower motor resistor is a variable resistor that controls the speed of the blower fan. It has several different resistances, corresponding to the fan speed settings (1-4, or however many your truck has). When a resistor fails, it often causes the lower speeds to stop working while the higher speeds still function. On higher settings, the fan might bypass the faulty part of the resistor, allowing it to work.
Other, less likely possibilities include:
* Blower motor itself: While less common than a resistor failure, a failing blower motor could exhibit similar symptoms. It might struggle to start at lower speeds but manage to overcome the problem at higher voltages.
* Wiring issues: A broken or loose wire in the blower motor circuit could also interrupt the lower speed settings. This is less probable than a resistor failure.
* Control panel issues: Although less likely, a problem with the climate control panel itself could cause this, but it would likely affect other functions too.
Recommendation:
Start by checking the blower motor resistor. It's usually located somewhere behind the glove box or under the dash, fairly accessible (though you might need to remove some panels). You can often find diagrams online showing its location in your specific Silverado model. If you're not comfortable working on car electronics, take it to a mechanic. Replacing the resistor is a relatively inexpensive and straightforward repair. Inspecting the blower motor and wiring is a good idea while you're at it if the resistor replacement doesn't solve the problem.
