DIAGNOSIS Yeah, this one's a classic. The printer's internal sensors are detecting something—often a tiny piece of torn paper, or a sensor flag itself is stuck—even if the main paper path looks clear.
FIX First, power the printer completely off and unplug it from the wall for a minute. Open all the doors: the front cartridge access door, the rear output tray, and the duplex unit (the flap on the back). Shine a flashlight inside and look for any shred of paper, especially around the rollers in the duplex unit and deep in the rear output area. Use tweezers to gently pull out any debris. Also, manually wiggle all the little plastic sensor flags you can see to make sure they're moving freely.
IF THAT DIDN'T WORK The issue might be with the paper tray itself. Remove the tray completely. Look at the metal paper length guide at the back of the tray cavity inside the printer. Sometimes it gets bent or has a sticker stuck to it, which tricks the sensor. Gently straighten it if it's bent.
HEADS UP Be really careful not to force any doors or levers when the printer is off. If you break a plastic sensor flag, you're looking at a service call. Also, using paper that's too light or has a rough edge can cause micro-tears that trigger this.