Even though I ran my car quite a few miles, the heater only blows out cold air. Why? It worked fine last week. PS. Do not share my email address with ANYONE!

sounds like a faulty sensor

Sounds like ur ac pump is locking up its time for another one and also is your coolant level okay maybe ur heater core went out they dont give early signs of quiting they just go out

If the car overheats also, replace thermostat.Could be heater core needs to be replaced, Any water on passenger side floor?, or smell of coolant?or are windows misty on inside of car?when you slide control from cool to heat it is supposed to close a door that lets in cold air, make sure thats working, usually you can hear it shut. lastly, check water pump. You do have enough coolant in system right?

sounds like your flaps are open.

You may need a tune up if its been longer than 12,000 miles since the last one.

Had similar problem. The actuator that opens & closes the air blend door quit working. This is easy to see if you remove glove box. The actuator will move when you adjust the temp selector (do this while the heater is running).

yes.bobg22

With the car warmed up,feel the heater hoses,are they both warm/hot near the firewall in the engine compartment?If they are both warm/hot,then the heater core should have circulation through it and it may be a "blend door" issue in the heater a/c assembly.If they are not warm/hot then the heater core could be dirty and plugged up and needs to be flushed.
If the above does not help,a drain and vacuum of the cooling system and vacuum refill will remove any trapped air.
Sometimes,clamping off the top rad hose and holding the engine rpm at 2000 rpm will force trapped air out of the heater core.(Watch temp gauge when trying this)
Lastly,a NEW thermostat doesn't mean its working correctly.

Please see
http://www.tundrasolutions.com/forums/highlander/32217-heater-quit-sort-of/
It's usually broken solder joints on a cable behind, and feeding, the temperature knob.
I repaired my 2002 Highlander yesterday thanks to this site.

Unfortunately, the A/C systems on the GMT400 (92-99) GM SUV's were not particularly reliable. A fuse may have blown (unlikely), compresser may be bad, the condensor could be out, or the entire system may need replacement.
My advice? Take it to a different mechanic, or a dealer.