Why does the printer keep asking for custom paper in tray 1?

The HP LaserJet printers might sometimes flash a message to the effect of "Place paper in tray 1." Tray 1 is the fold-down tray, not one of the two main paper trays, and is where one would normally feed non-standard paper. This message is displayed for two reasons:

1. someone has sent a print job with a non-standard paper size selected
2. someone has changed the paper types for tray 2 or 3 at the printer console.
   

If the problem persists for more than one print job, it is likely the latter problem and you should contact the system administrator.

In most cases, you can simply press the Go or Start button once or twice to force the printer to use the paper in the main trays.