HP LaserJet LJ 4350 locks up when duplexing

Symptom: Your LaserJet 4350 prints just fine until you try to print a double-sided page. Then you see the page start to exit, pull back in, then stop. The Data light continues flashing green, and the display reads, "Processing..." There is no error indication, but the machine won't print the second side and continues to hold the half-printed paper in the duplexer. If you open the top cover and close it again, the printer will eject the page, then try to print the whole thing again, unless you're quick enough to cancel the job first.

Cause: The printer is expecting a different sized paper than what it's running. Often it's not the menu settings, but a little lever in the paper tray itself. Pull out the paper tray you're trying to print from and look for a little blue lever on top of its right-hand side. The lever is labeled "Standard/Custom." Flip it to Standard (parallel to the tray side) and your problem should go away.