I have a 2001 sierra 1500 ext cab. Just recently had the coolant system flushed and now the heat is blowing cold air every so often. Not too sure what it could be. Anyone have any suggestions or have had the same issue?? I was thinking maybe the thermostat??? What do you think??

 

1999 gmc sierra 1500. no heat and defrost only/ no control

 

Blend door actuator or partially clogged heater core. A flush doesn't always unclog the heater core.