Yukon a/c blower doesn't work on low settings

The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

HVAC Blower Controls Circuit Description
The blower motor is a variable speed motor. The motor operates at a higher rate when voltage is increased to the maximum level.
The blower motor resistors reduce the voltage supplied from the A/C FAN fuse (LH IP Accessory Wiring Junction Block) when the blower control is set at low and medium speeds (speeds 1-4). The blower motor relay is energized when the Blower Motor Control is set to the High position. This removes the blower motor resistors from the circuit and battery voltage is then applied directly from the HVAC BLO fuse (RH IP Accessory Wiring Junction Block) to the blower motor though the relay switch contacts. The blower motor then runs at high speed.

sounds like blower bearings are dried out and will eventually lock up the blower..sometimes you can spray wd40 in the blower bearings and it will work fine for a while.. also check vacuum controls to the baffle dors in the ductwork..may have some leaks there.. should hear a slight hissing if really bad..repost findings

ok that means that speed number 5 on the fan blower is bad or your climate control unit is malfuntioning

1st thing to do is change the thermostat. It is either missing, or stuck open (you provide good info). This is why the car is not running at operating temp and why you have little heat. The no blower for settings 1-4 probably means a bad switch.

There is a resistor down below the blower fan on the inside firewall of the passenger side. Advance or Auto-zone carry them and they are not hard to replace or very expensive. You will have to have a small socket though. I am not sure it is a 6 or 8mm.

You will see a long plug about an inch and a half that goes into it. When they burn out you only get high as a default.

Hope this helps.

check the brake fluid level, if its low, there may be a leak, if the level is good, the master cylinder is bad.

replace the A/C control head... very simple to do. Pop off the dash panel and pull the control head out... unplug the 3 harnesses and there you go.

Check the fuse for that motor also check your radiator fluid level cause if its low than blower will not turn on. Happened to me about a two months ago

there is a ballast resistor that should be next to the blower motor that controls the low speeds on the blower. change it and you should be all set.

It could be the actual switch maybe???