GMC Repair: heater/defrost is not working, gmc yukon xl, 2000 gmc yukon xl

Question
Hi, I have a 2000 GMC Yukon XL 1500 it has the climate control on both dash and in the roof for the back heaters. The dash board heater and defroster are not working. It blows out cold air no matter what you do. The rear heater is working just fine. We were told it would not be a fuse because it is blowing air. My husband is trying to fix it himself but so far to no avail. It is really cold and icy here and without a defrost it makes it almost impossibly to drive. Do you have any ideas?

Answer
Hi
I would advice 2 items to check...
1) have him feel the heater hoses going into the dash after truck is warm... the hoses BOTH should be warm, 1 hot to touch, and the other cooler than the other.
If the hoses are both cool to touch, then the heater core is clogged or partially clogged.
Shooting LOW PRESSURE air through the core in the reverse (backflushing) will, sometimes, clean out the core to provide heat again.
2) If heater core checks out OK, then it is likely the temperature door actuator located on the HVAC case, directly above the "hump" of the floor under the dash.
It is abouth as big as a standard pc mouse, alittle thinner, and has 5 wires going into it.
*********You MAY try this first*******
unhook BOTH battery cables from the battery.
touch both cables together for about 30-45 seconds...
Then reattach the POSITIVE cable to battery first, then negative.
Then give it a try.....
I hope this all helps
Pawl