2003 Toyota Highlander AC/Heat problem

Need a little clarification, is the air just going cold after a second? Or is the controls going to cold on their own?

If the air just turns cold, but the indicators still show you are asking for heat. Start with the easy stuff. Make sure you cooling system is full and free of air pockets. (Eric has a great video on here, search for bleeding cooling system) If you have air in the system it is likely to get trapped in the heater core and prevent hot coolant from flowing through.

Make sure your truck warms all the way up to operating temperature. IF you truck does not heat all the way up there is not enough heat to allow an exchange in the heater core.

Sit in the seat and move your temperature control from hot to cold and feel for a change in the way the air blows through the vent. Even if there is no change in temperature you should feel the air intensity vary as the blend door moves across the air flow (and sometimes you can hear it move) If you don’t notice any change at all the motor could be bad or the door jammed or even the controller is bad.

Or you could have a clogged heater core, preventing flow and a proper heat exchange. One way to tell is if one heater hose going into the core is hot and the other is cold.

Might be some wiring problem. But, it’s always better to consult some technicians before touching any wire. I prefer air conditioning repair palm beach gardens center as they provide very quick and an economical service.