The front expansion valve is clogged.
Your heater will blow cold air only.
It doesn't really blow it out it just warms the cold air.
Most likely cause is low coolant. Check your antifreeze level.
Probably need to add antifreeze. Check the Radiator not the overflow tank. If you are low on antifreeze it will not circulate through the heater core.
it wont blow heat itll blow cold air
you need to replace your heater core
you are low on coolingliquid !
heater core possibly plugged and leaking
I'm in the same boat: Try reading all pages on these links: http://townhall-talk.edmunds.com/direct/view/.f15850c/0 http://forums.automobilemag.com/70/6248778/toyota/2002-toyota-highlander-problems/index.html this one has photos how to do the repair: http://www.tundrasolutions.com/forums/highlander/32217-heater-quit-sort-of/
No, not if it's on a heat setting. Most home thermostats have a "Fan" setting that will blow cold air if the attic is cold.
you may not have the heater water hoses plugged in