I have a 2000 Cavalier 2.2L. I changed the thermostat, and the temp sensor. I had heat at a stand still and would lose it while running 2400 or more (60+mph) and when stopped it would come back. I changed the heater core and now have no heat period and the losing heat issue is still here. Coolant level is good and its bled. Do I change the temp sensor again? Engine is hot under hood but have zero heat in the cab. Dont know what else it could be. I'm going to try the temp sensor again and a flush, or does anyone have any suggestions?

 

Kinda sounds like the thermostat is stuck open. Double check it & check its operation with a pot of water on your stove. After the water reaches thermostat Operating temp drop it in & see if it opens. After you remove it see if it closes back up.