My 08 Trail Blazer - there is no heat til I am about 30 miles down road. then a little heat comes out but fan does not start. Other days fan comes on but no heat. I replaced the rheostat twice and blower motor once. When I replace the rheostat it works for a day and then I have the same problem. As for A/C it works for a short time then fan shuts off cool air finally dies off.