DIAGNOSIS Yeah, this one's common. The electronic filter indicator didn't reset when you installed the new cartridge. It's a separate step people often miss.
FIX With the filter installed and the unit mounted on the faucet, find the small circular "Reset" button on the front of the housing, near the light. Press and hold that button for about 3 seconds. You'll see the light cycle from red, to blinking yellow/amber, and then settle on solid green. That means it's reset for a new filter's lifespan.
IF THAT DIDN'T WORK Make absolutely sure you're using a PUR PLUS faucet filter (like the PFM400C), not a standard PUR filter without the "Plus" technology. The non-Plus filters aren't compatible and won't trigger the sensor properly. Also, check that the battery cover on the bottom is securely closed.
HEADS UP The indicator is just a timer based on average gallon use. If your water is particularly hard or has high sediment, the filter might clog *before* the light turns red. Don't rely on the light alone if water flow slows to a trickle.