DIAGNOSIS Debris (sawdust, drywall dust, metal shavings) has packed into the switch mechanism. The switch slides a physical bar that hits microswitches inside; grit jams it.
FIX Clean it out.
- Remove the battery.
- Use compressed air to blast around the base of the switch.
- Spray a small amount of electrical contact cleaner (not WD-40) into the switch slot while working it back and forth.
- Let it dry completely before using.
IF THAT DIDN'T WORK If cleaning doesn't free it, the switch mechanism itself is broken or the internal plastic slide is cracked. This requires replacing the entire switch/trigger assembly, which is sold as one part.
HEADS UP Never force the switch. A switch stuck in the middle breaks the electrical connection, which can mimic a dead battery or trigger failure. Forcing it can snap the internal plastic lever.