DIAGNOSIS The variable speed switch or its control circuit is malfunctioning. It's stuck in the high-speed mode.
FIX This is almost always a switch failure, not a user-fixable issue. Try cleaning. Remove the battery. Spray electronic contact cleaner (like CRC QD) into the switch button seam while pressing it repeatedly. Let it dry completely.
IF THAT DIDN'T WORK The internal switch assembly is defective. The DCS354B uses a sealed electronic switch module. It requires replacement of the entire switch/trigger unit.
HEADS UP Do not disassemble the tool housing to access the switch unless you are qualified. You will void the warranty and risk electric shock from capacitor discharge.