Heater blowing cold air. like air cond. on. i changed thermostat

Check heater tap is operating remove heater hoses and back flush the heater core to check flow, check thermostat.

Most likely the thermostat broke. I expect that you have an older Hyundai and never had the anti-freeze changed. When the engine is cold the thermostat is closed which causes the engine to warm up more quickly. A warm engine runs more efficiently (less gas) and more smoothly. When the engine temp reaches a certain temp the thermostat opens and then allows the hot engine coolant to run through the lines to the heater core. The heater blows air over the heater core which produces the warm air that warms up the cabin. Thermostats used to be cheap and easy to change. I would suggest a web search for how to replace the thermostat. Something like 'hyundai accent 2002 replace thermostat".

Replace the thermostat.

Temp is controlled by an electric motor(or could be vacuum hose) on the blender door of heater housing. If you find this is operating correctly, then the problem is a plugged heater core.Backflush it by removing both heater hoses and flushing from water pump side toward thermostat side.don-ohio

Probably a stuck open thermostat or the thermostat is the wrong temp range for your vehicle.

Heater core could be blocked up, or the blend door is stuck on the cold side of the a/c evaporator or the blend door is stuck in the cold position.

YOU MUST HAVE THE TEMPT BLEND DOOR TESTED BY THE DEALER,AND OR THE MATRIX DOORS HAVE SYUCK.

my 2000 saturn sl blows cold air with the heater on. I changed the thermostat,and checked out my heater core.Still blows cold air.

Change thermostat if that doesn't solve the problem then its probaly a bad heater core

check thermostat and coolant level