I have an '03 Highlander. Although I haven't had the dash apart, I think the heat and AC is all controlled electronically. By that I mean there are no cables connecting the knobs to a valve or duct door, those knobs set by an electronic signal to a motor that opens the ducts or in your case opens and closes a valve allowing coolant to the heater core. Personally I don't think opening up the dash will allow you to find any parts that can be replaced easily, I think you may have to bring it to the dealer for a diagnosis. It may require replacing the electronic controls in the system or the small motor that positions the water valve on the heater core.
|