More information would be needed to diagnose your problem more accurately, but here are some tips to start:
1) Fans will not work with dimmers. If you have a dimmer controlling your fan, it will only work when the dimmer in is the full intensity position (if at all). You will need to replace the dimmer with a fan speed control. If one switch controls both the fan and the light, this will not solve your problem, as a light will not work with a fan speed control switch. You will have to settle with a standard single pole switch, a remote switch, or get one more wire between the switch and the fan.
2) If you have a fan speed control switch, but the fan doesn't seem to respond as it should, you may want to pull on the pullchain directly on the fan. If the pullchain is not set to high, your speed control will not work correctly.
3) If #2 or #3 don't apply to you, and you are simply using the pull chain on the fan, you may have a bad switch inside the fan housing, loose or disconnected wires to the fan (unlikely), or simply a bad fan motor.
The 2004 Chevrolet trailblazer air conditioning fan switch has three settings. The switch can malfunction, and only work in the high setting.
Had one that the fan motor was going bad Was burning out resistors with high draw
blower motor resistor
The fan control module in the engine compartment has gone bad.
If only high speed is effected check the high speed relay and/or the fan switch/electrical connections for problems.
the blower motor resistor is bad or the connector going to it
You need to replace your Blower relay,
try replacing the blower motor resistor
the blower resistor controls the fan speeds
Bad resistor or a malfunction in the switch.
Defective Heater Resistor Pack.
Check the low speed relay and the resistor at the motor
blower motor resistor needs to be replaced
You most likely have a bad blower motor resister.......
You most like have a bad blower motor resister.......
The switch is probably fine. The culprit is the resistor unit it is mounted in the air duct near the fan
do you mean the speed of the fan? if so, it'a relay generaly near the motor of the fan. If it's the heat who only came at high speed, you need to add some antifreeze.
from what i have read it is the blower motor resistor it goes bad and only lets the fan run on high google blower motor resistor and read on
Blower Motor Resistor is bad. 20.00 part at AZ.
replace the blower motor resistor
If you mean the fan - replace the fan speed resistor if the fan only works on high speed. Please supply more information - no heat or cooling? Does fan work? Does mode switch work?
Odds are you need a fan switch, It is very common for the high on GM fan switches to go out.
The fan switch is probably bad. My Ford Explorer did that, I had to have the switch replaced. This could also be caused by a bad blower motor resistor.
It sounds like your blower fan resistor is not working correctly. Eventually it will stop working all together. I suggest that you get a replacement.
There is a fan motor resistor board located under the dash on top of the plastic duct housing on the passenger side of the vehicle. It is behind the glove box. Removing the glove box will help you to get to the resistor. The power to the fan motor uses various resistances on that board to get your lower fan speeds, but on the highest setting it bypasses the resistors since they are not needed. You always will get your blower motor to work but only on high only when the resistor board is bad.