DIAGNOSIS The brushless motor controller is detecting a fault, most likely a stuck or failed trigger switch assembly. The single click is the electronic brake engaging.
FIX Clean the trigger mechanism first.
- Remove the battery.
- Spray electrical contact cleaner (like CRC QD Electronic Cleaner) into the trigger slot while working the trigger back and forth.
- Let it dry completely for 10 minutes before reinstalling the battery and testing.
IF THAT DIDN'T WORK The trigger switch itself has failed and needs replacement. It's a common wear part (part number 191695-2 for the switch assembly). You'll need to open the drill housing to swap it.
HEADS UP Don't use compressed air to blow dust out of the trigger—it can force debris deeper into the switch contacts. Use contact cleaner designed to displace moisture and dissolve grime.