DIAGNOSIS The base’s bag-full sensor is dirty or the bag isn't seated correctly. The sensor uses a small infrared beam inside the base; dust blocks it.
FIX Unplug the base. Remove the dust bag. Use a flashlight to look inside the bag port. You'll see a small clear plastic window on each side. Wipe both windows with a dry cotton swab. Reinstall the bag firmly until it clicks. Plug the base back in.
IF THAT DIDN'T WORK The sensor itself may have failed. Unplug the base, wait 60 seconds, then plug it back in. This reboots the base's controller. If the error persists, the base likely needs replacement.
HEADS UP Never use compressed air to clean the sensor windows. It can force dust deeper into the mechanism.