The servo is probably stuck. They all go bad eventually. In time even the best metal-body rebuilds will fail. It's a genetic defect in the servo design. Even if you don't need heat fixing a dead servo will be a + because you'll get back all the fan speeds and the AC will work a lot better.

A rebuilt servo isn't very expensive and it's not hard to install. You can also get rid of the servo completely and upgrade to a digital controller.

Steve
380SLC
240D
300TD
E300DT