We dont have a lot of those controllers in my neck of the woods but I have had sone of those on my routes in the past. Somehow I remember stm being related to either the drive, encoder or speed sensing related. Im pretty sure those controllers have the speed boards in the back top right of the controller. There is also a speed relay on the right I believe. Make sure the relay contacts are good. If you have a speed board switch it with one from another controller and make sure your encoder coupling is good.