DIAGNOSIS Almost certainly. The light is powered by a single 4LR44 (or equivalent) 6-volt alkaline battery housed in a compartment on the bottom of the filter head.
FIX Unscrew the filter head from the faucet. On the underside, you'll see a small rectangular battery cover. Use a coin to turn the screw and open the cover. Replace the old battery with a new 4LR44 6V battery, ensuring the positive (+) terminal faces the compartment's center (usually marked). Close the cover tightly.
IF THAT DIDN'T WORK Check for corrosion on the battery contacts. If you see white or blue crust, clean it off gently with a cotton swab and a bit of vinegar or rubbing alcohol. Also, ensure the new battery is fresh and seated correctly.
HEADS UP The filter will still work perfectly without the light—it just becomes a manual-change system. The battery typically lasts through 2-3 filter changes (about a year). A dead battery doesn't affect filtration performance.