Heater isn't working

well, could be 1 of 2 things. there might be an air lock in the engine, or the heat controller is not switching from cold to hot. hopefully that helps out.

could be that your heater core is blocked or the flaps that control heat/cold is stuck in cold position.

I did have that problem with my GMCJimmy 2001. I have to flush the system and after that remove the hose of the heater core and put water with pressure to clean it up.