DIAGNOSIS This is a failure of the variable speed trigger potentiometer. The internal component that registers trigger position has failed, sending only an 'on' or 'off' signal to the controller.
FIX The trigger switch assembly (02-06-2821) must be replaced. The potentiometer is integrated into the switch and is not a separate serviceable part. This requires disassembly of the tool's handle.
IF THAT DIDN'T WORK The issue could be in the motor controller board. If the controller receives a good signal from the trigger but does not regulate motor speed, the controller is faulty. This is less likely than trigger failure.
HEADS UP Operating with a stuck-on full-speed trigger is a safety hazard. The tool can start unexpectedly. Remove the battery when not in use until repaired.