Heater and defrost or floor vents dont work.

ID Status Date Year Make Model Transmission Type A/C Controls Public/Private
#15861 Closed 2001 GMC Yukon xl 1500 slt public

Ok so it’s a 2001 GMC Yukon xl 1500 slt. It’s got the temperature controls with the degrees in farenheit and rear defrost button on the control head/module. I have ice cold ac but I have no heat and no other vents blowing air. Meaning if I change it to defrost the windshield or blow air at my feet it laughs at me and says no. I’d rather not throw parts at it since Christmas is around the corner. Could you guide me to testing the controls and checking the actuators and identifying them or even a chart to which 1 is which. Thanks in advance for any help sparky.


I recommend performing a system recalibration.

  1. Starting with the ignition off. Remove the Radio fuse for at least one full minute.
  2. Re install the Radio fuse.
  3. Turn the ignition on and wait 4 full minutes. Do Not Touch anything during this time.
  4. Turn the ignition off for at least 10 seconds but no more than 30 seconds.
  5. Start the engine and check the HVAC operation to see if the condition is corrected.

Ok I pulled the fuse waited 2 minutes reinstalled it and turned my ignition switch to the on position waited for 4 minutes turned on the truck and still no heat or different vents on the front.


Did you cycle the ignition off for at least 10 seconds but no longer than 30 seconds after the ignition had been left on for four minutes? If so my next question would be did you lose temperature and vent control at the same time? Have you checked the engine coolant level. If it is down any the heater core may be empty while the engine has enough coolant to function properly.

Yes I cycled it. The vents went I still had heat then I lost heat. Coolant levels are fine. I’m thinking it’s electrical but I can’t find how to test the control head.


I always try to start with the simplest testing which in my case would be to look at codes and data with a scan tool. I am assuming you do not have a scan tool so my recommendation would be to remove the temperature door actuator and turn the door shaft with your fingers to see if you can get heat out of the system. Of course the engine does have to be fully warmed up to check this. By doing this we can either confirm or exclude an electrical fault. Also, if you use a flashlight and look carefully at the actuator shaft while someone else adjusts the temperature controls you may be able to see if the door is moving or not. If the actuator shaft moves but there is no heat then either the door shaft is broken internally or there is a problem with coolant flow through the heater core. If the actuator shaft does not move then you would need to do some voltage testing at the actuator harness connector. You would need a good multimeter and some small pins to do backprobing of the wire terminals.

I’ve got a fluke multimeter and an autel maxi check pro scan tool. Only codes that show are the downstream o2 sensors saying there’s no voltage to them. I’m 99.99% sure this is electrical or a bad actuator. That’s why I was asking how to test them specifically. Everything was working but then the vents didn’t switch anymore for the defrost or floor then I lost heat. I figure it’s almost 20 years old and the control head/module is on average $200 for a replacement and the actuators are about $40 each that I should try to test them between voltage readings and finding out which is which on the actuators. I understand that everyone is different from basic knowledge and what not. I just want to get to this test to make sure that 1 I don’t shotgun parts at it and 2 I dont waste money since the parts store does not sell test parts. And thanks for responding on Thanksgiving hopefully you have a good Thanksgiving.


1. At the blend/temperature actuator check for Battery Voltage (B+) at pin 5 Brown wire, a good ground at pin 7 Yellow wire, 5 volts reference pin 10 Light Blue/Black wire, and a voltage that will vary between under 1 volt and over 4 volts (feedback) pin 9 Light Blue wire (there will only be a voltage change if the actuator moves)

2. Check for 2.5 volts at pin 6 with the actuator not being commanded to move. When the actuator is commanded to move, the voltage will change to either 5 volts to move the actuator one direction, or 0 volts to move the actuator the other direction. Once it has reached the commanded position or the computer has determined a stall condition (could take two minutes for a stall condition to be determined by the software), the voltage will go back to 2.5 volts.

3. If there is no command from the control head, perform a recalibration and monitor the voltages during the recalibration time frame. It may be necessary to perform a hard reset of the system by removing both battery cable ends and holding them together for one full minute. You would need to treat reconnecting the cables to the battery as the installing of the removed fuse step in the recalibration procedure. Cycle the ignition on and off per the recalibration procedure from this point. Keep in mind that all learned memory will be lost (clock, radio presets and memory seats). Also the idle may be erratic and the engine may stall until the throttle body is cleaned using the appropriate cleaners and techniques. 

4. If the actuator moves, backprobe the Light Blue wire and expect a varying voltage. If the Light Blue wire shows 0 volts or 5 volts, replace the actuator.

Write a reply

The ticket has been closed. If you feel that your issue has not been solved yet or something new came up in relation to this ticket, you can re-open it by clicking this link.