My 2004 Hyundia Elantra is blowing cold air when I have turned the heat on. Someone I know removed the hoses and put an air gun, thinking the hoses were clogged, heat worked really well for about 45 minutes and then went cold again. Next he changed the thermostat for me, no luck on that. Can someone please tell me what is wrong with my car. At 25 degree mornings I am desperate to know. I just hope it is not the most expensive answer. (The Heater Core) Is there a fuse or switch that controls the heat. I do know that only one hose is getting hot and the other remains luke warm.