One day I tried to fill my dell 3100cn color laser printer, but the "Load Tray Letter" error wouldn't go away. I checked all of the settings, made sure the blue bars in he tray were properly clicked in, tried clearing the nvram, but I couldn't even get it to print the diagnostic pages (turn off the printer, power up holding up and down arrows to get into diagnostic mode). I could print using the MPF tray, so I decided to use that for a few months until I finally got fed up with the error, and decided to figure out what was causing it. As it turns out, it was a mechanical failure, and a very simple fix.
If you look closely at a working 3100cn you might notice that inside the tray there is a metal plate on some springs that pushes the paper up into the feeder mechanism. When you pull the tray out there are some catches that compress the springs to allow you to pull the tray out, and when you push the tray into the printer there are some small plastic clips snapped into the bottom of the printer which release the catches. On my printer one of the clips was completely broken and floating loose in the printer, and one looked like it was about to snap, so I used some 5 minute epoxy to put them back into their corresponding positions--and viola the error cleared. Not bad for a printer sub-$500 printer with over 13000 pages on it
It is easy to see if your printer needs this repair made, simply pull the tray out completely, and look inside where the tray came from. Near the back of the metal plate that is the bottom of the printer, there should be 2 white plastic clips (you might need a flashlight to be able to see them). If either of them is missing or damaged, then this fix should apply.
A picture of the base plate, and a detail of the clip.
If you do find that one of your clips is damaged, here is the fix:
The repaired clip, and a view from the bottom of the tray mechanism.
I received an e-mail from Kevin:
On the front of the printer, when you pull out bottom tray, you'll see three small, white buttons. Their mounting is poor and if they come loose, you'll get the same symptom. The fix is simple, snap the fixture back in place (to the right, toward the tray).