This is a very well documented problem many Highlander owners have. I have an ’05 and the same thing happened to me. The problem is that when they manufactured these heating/ac units the wires that are soldered on the circuit board behind the middle knob are too short and because of vibrations and twisting of the knob from hot to cold the wires break off the circuit board.

Sometimes people including dealers claim to have fixed this problem by removing the knob and tightening up the nut behind the knob. This will appear to have fixed the problem because tightening up the nut puts pressure on the broken wires and they make a connection but it’s not actually fixing the problem which is the broken wires, so after hitting a few bumps in the road and twisting the knob from hot to cold several times the heat stops working again.

You can fix this problem permanently and it will turn out to be better than it was originally. First pull the whole unit out of the dash board. Remove the back of the unit. You will notice the broken wires on the middle knob. You need to CAREFULLY solder the wires back onto the circuit board by either getting longer wires or cutting down the middle of the 3 existing wires with a razor blade to allow for more slack. If you do a sloppy soldering job you will end up with just normal air (No heat or ac) so make sure there are 3 distinct connections that don’t touch. Finally using a hot glue gun, hot glue a couple of blobs connecting the circuit board to the cheesy plastic clips that aren’t doing their job very well. The whole reason the wires broke in the first place was because of the loose circuit board and no slack on the wires. I fixed mine and it works great. Use the smallest soldering iron you can get.

These things cost over 800 bucks to replace and your "new" one will be just as crappy as the first one. This fix is better than new and will never break again!

There's nothing like driving down the road in Northern Indiana with a wind child below zero and have no hear in your car. It's like drive your freezer down the road. We are using the recirculating air button to get some our own warmth back into the SUV. I have read other owners are having the same problem and the dealerships are charging $150 to diagnose the problem and up to $1,100 to fix when it might be as simple as turning a nut. I am having my husband check out if it's a simple fix or what. I am angry and come home with very cold fingers and toes every time I have to get into this car!!