"I just picked up an HP DeskJet 520 printer that doesn't work. On startup, the print head moves right an inch, then all the way left, where it slams into the left side of the carriage and grinds away for about a tenth of a second before stopping."
(From: Tech Guy (firstname.lastname@example.org).)
You may wish to check the undercarriage (no pun intended).
The printhead location sensor microswitch may be on the fritz The printer uses this switch to determine the starting point of the printhead after which it uses assumed location by how far the data has sent the head every time the unit gets a reset code, it checks this switch and if the signal is not detected, it may slam the head to either rail end or not move at all. If this is the case you can make sure that the platen is not clogged with label or paper debris. Gently move the head by hand to the right. If you shine a bright light into the area where the printhead usually calls "home" you may be able to see this switch (it may however be located under a cover triggered by the belt) if the switch is defective, replace it. If in fact it is jammed by debris, simply clear it and you may have solved the problem. Beyond this, you may have a logic problem (bad chip or other component) I make a good practice of doing a thorough cleaning of all machines that have left my shop to reduce the possibility of other problems during my warranty period. (it also makes the customer think that they have gotten something for their money)
I have replace a switch or a fuse on many machines, charged my base fee and heard the response upon their pick-up by customers that, "I can tell right away that you have found and fixed the problem" without even so much as a demo.
There is a clear plastic strip strung between the both ends of the printhead pathway. This strip of plastic has microscopic vertical bars which the printhead can read and sense what position its in the pathway. The strip may be contaminated with excess ink which may confused the printhead. Take a soft cloth or Q-tip dampened with water and wipe of the strip (the ink is water soluble) and the printer should work.
(From: Raymond Carlsen (email@example.com).)
Closely examine the toothed belt that drives the printhead. Look for a few missing teeth at one end. I managed to make one work again by shifting the belt over a bit (past the bad teeth). If that's it, the belt should of course be replaced.