Heater blows cold air

bad heater control switch or resistor.

HEATER NOT BLOWING COLD,NO HOT AIR FROM HEATER,A/C BLOWS HOT AIR,A/C NOT BLOWING COLD, COLD AIR FROM HEATER,BLEND DOOR ACUTATOR CLOSED,HVAC UNIT NOT OPERATING,
http://alltypesofpartsreplacing.blogspot.com/2012/12/heater-not-blowing-coldno-hot-air-from.html

Check the heater tap moves when you adjust the temp dial. A/C will not work with the heater on.

the flapper door is not closing and opening properly...it is located in the duct work....remove heater control panel and follow the cable to the flapper door

If it was a fuse it wouldn't blow at all. Check coolant level. Change thermostat. If still not working possibly a heater core - symptoms of a bad heater core would be losing coolant, fogging of inner winshield with defroster on or wetness on passenger side floor.,

CHECK COOLANT MAKE SURE ITS NOT LOW.IF COOLANT GOOD THERMOSTAT BAD.

There amy be a problem with the heater relay switch which is not allowing it to switch over to heat.

first check and see if it has a thermstat installed or stuck open, second, make sure that your heater has not been bypassed by connecting the two hoses together

maybe the heater matrix could be at fault, thats the bit that controls the temperature in the car.

on the evap box there is a control mtr that switches the hot cold door. these mtrs are prone to failure. the other problem that i have seen is that the door breakes in side the evap box and that means removing the box to replace the door . if i remeber correctly you may have to slide the dash back to gain assces to the control mtr to test it good luck