2001 BLAZER LT TURN ON HEATER ONLY COLD AIR WHAT SHOULD I CHECK

 

thermostat maybe?

 

well, only check the thermostat if it is slow to come up to operating temp (around 200*F - tstat is 195*F actuated). If you are getting no heat inside, first check both of the heater hoses and make sure that you are getting hot coolant into and out of the heater core. If both hoses are the same temp, then your heater core is mostlikely ok, but if one is measurably warmer than the other, then you may have a clogged heater core. Best way to remedy that situation is with a reverse flush. Unhook both hoses on the opposite end (away from the firewall). Remember which hose was the cold one and stick a garden hose into it. This should flush out any of the crap that could accumulate in the heater core. Keep the hose to it until clean/clear water comes out. If water won't flow through the core, or it flows VERY slowly, then you need to look at replacing the core ($$$).

Now if both hoses are the same temp, then you may have a problem with the heater door in the HVAC housing. The doors are run off of vacuum and sometimes you can either get a hole in the actuator diaphram or the hose could pop loose from either end. I am not sure if there is a bypass valve (that would make it bypass the heater core), but if there is, then that could also be a problem area.

Good Luck