DIAGNOSIS Reduced power output. This is typically caused by worn motor brushes or a failing field controller, not the battery.
FIX Confirm it's not the battery. Test with multiple known-good, high-output M18 batteries (e.g., XC5.0 or HO). If performance is consistently weak, inspect the brushes. On the 2904, this requires disassembly. Look for brushes shorter than 1/4 inch or springs not making contact.
IF THAT DIDN'T WORK The electronic variable speed trigger or the power module may be failing, sending incorrect current to the motor. This is not user-serviceable. Tool needs professional diagnostics.
HEADS UP Using the tool in hammer mode on concrete for extended periods generates extreme heat. Chronic overheating is a primary cause of electronic component degradation and power loss.