Your battery is likely not making proper contact, or you're using a battery not specified for this tool. The saw is designed only for BL1815N, BL1820, BL1830, BL1840, or BL1850 battery cartridges. Using a different one is dangerous and won't work.
- Remove the battery cartridge.
- Check the battery terminals and the saw's terminals for debris, dirt, or corrosion. Clean them with a dry cloth.
- Firmly re-insert a compatible battery until it clicks into place.
- Ensure the switch is in the off-position before connecting the battery. The tool must not start when you insert it.
IF THAT DIDN'T WORK: The switch itself could be faulty. A power tool that cannot be controlled with the switch is dangerous and must be repaired. Do not attempt to bypass it.
HEADS UP: Never store the battery where it can short-circuit. Keep it away from other metal objects like nails, coins, or keys. A short causes large current flow, overheating, and possible burns or fire.