According to HP, when the requested paper size from a print document does not match the size that the printer is sensing, the printer will prompt you to load the requested size paper in the upper, multipurpose Tray 1.
If the HP LaserJet 4000, 4050, 4100, 5000 or 5100 series printer displays the message "Tray 1 Load Plain Letter " or if the paper is being pulled from an unexpected paper tray, try the following steps
Follow these steps to check that the paper guides in the trays are correctly configured.
1. Remove the tray in question from the printer, take out all of the paper, and verify that the rear paper guide and the side paper guides are aligned with the "Letter " guide slot and not the "A4" guide slot.
2. On some paper trays, there is a dial on the right of the tray, along the outside area, where the paper is loaded toward the front. Follow these steps if the tray has the dial:
a. Pull the tray out of the printer.
b. Look at the blue paper-size-adjustment-dial and verify that the correct paper size is selected
In addition to HP’s solution to this error I have discovered another cause for this error message. Look at the right side of the tray from the outside and see tabs that activate switches inside the printer. These tabs are activated by sliding the blue plastic backstop which adjusts for paper size. This blue adjustable backstop moves a white spring loaded plastic bar that pushes on the tabs. On one printer I recently repaired the white plastic bar was behind the blue adjustable backstop. This restricted the forward movement of the white strip preventing it from activating the tab.
Without the tab activated the printer could not recognize the letter size paper in Tray 2 and called for Tray 1. To repair this I disconnected the plate the paper sits on and moved the white bar to be totally in front of the blue backstop. The white bar activated the tabs which made the switches allowing the printer to use letter paper in Tray 2.