like the Q says ac and heat quit working on my 03 durango does nothing at all on high works on all other settings.
most likely it is the switch on your dash that is the problem.
Sounds like an element in the resistor trio is burned out, though I'm not sure on the '03 where they put it. If you can see the motor from under the hood on the firewall, it's usually right next to it, it has electrical wires attached, and often held in by two screws. Good luck!
Best of luck: If you don't feel comfortable installing it get a mechanic to install it or diagnose the problem.
