DIAGNOSIS The red/green flashing LED indicates a communication error between the tool and the M18 REDLITHIUM battery. The tool has detected a potential fault.
FIX Perform a reset. Remove the battery. Press and hold the trigger for 30 seconds to discharge capacitors. Reinstall a known-good, fully charged battery. If the error clears, the original battery may be the issue. Test the original battery on another M18 tool.
IF THAT DIDN'T WORK The fault is likely in the tool's battery terminal interface or main control board. Clean the battery terminals on the tool and battery with a dry cloth. If error persists, the tool requires service.
HEADS UP This flash pattern is different from a simple low-battery warning. A flashing red light alone indicates low battery. Red/Green flash is a tool/battery communication fault.