DIAGNOSIS The variable speed control circuit in the trigger is failing. This is often due to dust ingress or a fault in the potentiometer (the part that measures trigger pull). The tool defaults to a safe full-on/full-off mode.
FIX Unfortunately, the fix is the same as for a completely dead trigger: replace the entire switch assembly (part 413038-6). The variable speed mechanism is integrated into the switch and isn't separately serviceable. Follow the same disassembly steps as the first fix—housing off, photo the wires, swap the unit.
IF THAT DIDN'T WORK In rare cases, the problem could be in the main control board that receives the signal from the trigger. If a brand-new switch assembly behaves the same way, the logic board is likely faulty. This is a more expensive repair.
HEADS UP When you have the housing open to replace the switch, take the extra minute to thoroughly blow out all the sawdust from inside the handle and around the circuit board. Preventative cleaning can save the next component.