So, I have a 1999 Blazer, 4wd, 4 doors, 4.3L V6, and it started blowing cold air. I noticed the temperature gauge was only reading like 140-150. I changed out the thermostat. That didn't do anything to change the temperature reading or the temperature of the air that was blowing out when I turn the heat on. What's the next most likely problem? Heater Core? Thanks for your opinions, I appreciate it.

 

99's are bad for the blend door actuator messing up. make sure the door is moving when you switch from heat to cool, you can check visually by opening the small access hatch ontop of the airbox. Circled here -

 

Welcome to the forum.

What do the 2 heater hoses feel like at the firewall when the truck is warmed up?

George