If it's not the rheostat or the knob then it's some other part on the electronics board. You can get the replacement electronics board as a whole from Festool, but this is expensive to replace, $141.
You can check all available Festool spare parts and diagrams at their EKAT catalogue
I'd check to see if I can find another faulty part on the electronics board and replace it myself. Perhaps it's only a faulty capacitor somewhere. Otherwise the only solution is to replace the entire unit.