So I have a 2005 Colorado. The AC worked intermittently but always only on high. One hot week it finally went out altogether. I did the research and replaced the resistor (it had burned out) and harness. Turn the car back on, still no fan. I let it rest, and a couple days later, poof, fan comes back on, but again only on high. It’s been like that since then (2 weeks or so). Any ideas what I need to try next to get the other speeds back.
I presume that the bearings on the fan are shot or some foreign object is in contact with the fan cage.
Either of these will over heat the resistor pack and even burn up the harness.
Replace the fan itself.
Yosemite
The problem might be with the blower motor relay.
When you switch the blower speed to high, the relay allows full voltage to the blower motor
When you switch to the lower speeds, relay then sends the voltage thru the resistors for the lower speeds.
Tester
Could also be a faulty speed selection switch or controller. An auto-electric specialist could probe the various parts of the circuit and tell you for sure what the problem is. If you wanted to replace the part yourself at that point you could. But guessing what it is and replacing parts on trial without some diagnostic voltage probing might be pretty expensive.