Why is my heater alternating between cold and hot?

it will need service,
check pressures.
hot air as in heater on, so turn off the heater,
test in pure A/C mode Cold.
when both A/C and heat are on this is called defrost mode
which adds cold dry air to hot air and dehumidifes the hot air and window.

Great question dustin but the A/C is suppose to come on in defrost or mix mode- this is floor & defrost. Does not seem to make sense does it. One of the things the a/c does is remove the moisture from the air. You need warm DRY air to remove the moisture from the windows. An automotive H.V.A.C. ( Heating,Ventilation,Air conditioning) first forces air thru the a/c evaporator core then thru the heater core and out the selected vent. If you only want a/c a blend door will by-pass the heater core which is controlled by a hot/cold knob or digital temp cotrol on the dash. Now back to the cold air problem- assuming the basics are all ok- Coolant is full, Engine gets up to proper operating temp-around 195 degrees- upper radiator should be hot. Both heater hoses should be hot. If thats all OK the blend door might not be working. It is controlled by an electric motor called a Blend Door Actuator which are prone to fail. I have replaced many on ford vehicles. Hope this helps

This may be due to a bad heater core or dirty core with a plugged water drain, first see if the a/c works. turn it on and see if the air gets cold and if the ac compressor works.

The way the defrost works to keep windows from fogging up is that when the front defroster is tuned on the ac should also be on, on most cars when you go to defrost it will turn ac on automatic. the system then uses hot and cold air to remove the moisture in the air to avoid window fogging. if your a/c is not working then that may cause window to fog, but if you are also getting a smell then the drain that lets out condensation may be plugged and then water builds up and also mold from always being wet and when heat is on you smell that.


i want you to run the defrost and when windows fog up wipe window with hand and see if it is greasy . if it feels oily then you may have a bad heater core, the core leaks coolant out and steam and causing windows to fog up with a flim of grease.

Does it sound like the noise the turn signal makes? Does the noise change if you change the speed of the blower motor? Do all the function of the heater work (defrost, vent, floor, recirculate, fresh air, hot, cold)?

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Feel both your heater hose near the firewall. If one is colder than the other your heater core is bad.
If they are both hot its in the cab controls. Lee

Sounds like defrost door on heater box is not work You need to get under dash and check controls on the box. Good luck

leaking heater core. The coolant level may be low also. What it is doing is fogging up the window with the steam coming from the hot engine coolant that runs through your dash board when the heater or defrost is on. This should be under some kind of warranty on an '08.

It sounds like you have a blend air door stuck. This is in the housing of the heater a/c blower unit under the passenger side of the dash. There is a motor on the top of the housing that could possibly be stuck or on working.

the blendor actuators strip out the gear inside the actuator cases on these all the time. replace the actuator.

I would think the door you hear flapping is the door that controls the airflow across the heater coil - and the fan keeps blowing the door closed, preventing the airflow from going across the heating coil. I don't know whether this vehicle is vacum controlled or cable controlled - so either it's a disconnected cable, or a leaking vacum line.