Hi, i have a 04 Blazer, bought it about a month ago and it has had a heater problem since i got it. all the vents work in the right pos, and the fan works very well but i have no heat. it turns out that the A/C was always on and i was told that it was the relay...ok no prob. i unhooked the compressor until i can get another. still no heat! i have been told that it could be the control module, or the vacume to the back of it, oh and i did replace the thermostat still blows cool air, not cold...just cool air no heat. any thoughts on this would be a GREAT HELP!
Thank You.

 

Changing the thermostat doesn't do anything for heat unless it's stuck in the open position in which case your car wouldn't heat up. At least in my 99 water appears to circulate through the heater core in a separate path and does not require the thermostat to be open. Check your heater hoses at the firewall. If they're both hot then it's most likely your blend door or possibly the automatic climate control if you have one. If you do have ACC put the dial on the red mark which I think bypasses the ACC and if your blend door is working and your heater core isn't clogged you should get heat. If your heater core is plugged you can try to clear it by backflushing with a garden hose or air compressor. It's written that you can safely go up to 50psi without blowing the core apart. If it is your blend door there are a few guides here and other sites describing how to fix it. BTW..I'm not a mechanic..just an owner who has had a lot of heating/cooling problems the last few years.

 

Thank You!

 

i tore the dash apart to check on the heater control mod..and found it was just fine, so the next thing was to blow out the heater core...i was hopeing like crazy that it would work cause i didnt want to tear the dash completely apart to replace the core. Any how long story short i blew the core out with a compresor and hooked the core back up and Wa La! i have Heat!

 

*Combining two threads concerning the same issue.*