Error message "Clear paper/document jam then power off /on"
There is no paper jam, I've tried cleaning every part, and still get
the message.  I've powered off, unplugged, disconnected everything,
reconnected, made sure the cover closes well.  Poked around inside &
didn't see or feel anything loose or out of place.
I believe it is a mechanical problem.

Hi mmyjuly,

Did you clean the optical encoder strip with a damp cloth? I found the
following Google Group discussion:

Question:
"I have a HP OfficeJet 590 that came in wtih two weird problems. One
is
that the printer will cycle, and then say 'Clear Paper/document jam,
then power Off/On.'"
Answer:
First thing to try is to clean the optical encoder strip with water
and soft
cloth."
http://groups.google.ca/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=jv33luku26j5sa5genfpqtf213nbu1krpb%404ax.com&rnum=1&prev=/groups%3Fnum%3D100%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26q%3D%2522Clear%2Bpaper%252Fdocument%2Bjam%2Bthen%2Bpower%2Boff%2B%252Fon%2522%2B%2B%2522%2BHP%2BOfficeJet%2B630%2522%26btnG%3DGoogle%2BSearch

I hope this helps as it sounds like a frustrating problem. If so,
please let me know and I'll post it as an answer.

Thank you,
hummer

Hi.  Thanks for your help.  Yes I did, but I cleaned it again, and it
was still a little dirty.  I cleaned it again with a soft damp cloth,
front and back, gently.  I went into the corners in the back with a
damp clean q-tip.  Now it's really clean.  I let it dry on both sides
while cleaning.  Then started up again.  I still have the error. 
There is a rattle on the right side where the cartridges go to be
lifted, or whatever it's doing.  The rattle used to happen when the
printer was out of paper.  Now it's happening everytime along with the
error message.  Here's the sequence:  Turn on printer, rattle under
the mechanism on the right side, the cartridge slides over to sit on
top of it, it lifts up and connects with the cartridges, the c. slides
back to the left, and stops, I get the error again.

Also, now at the very beginning, it gives me the message "Press START
to reset the date or STOP to skip"  It only gave me that message when
I unplugged from the power source, waited 15 seconds, then replugged. 
Now I'm getting the msg with only the on/off switch.  I've tried both
ways (resetting the date and STOP) with the same results, now I just
press STOP.  Following that, I just got Error 3409, a motor error.  I
just pressed "off" and the 3409 error cleared, I wait 15 sec., then
press "on" again.  I get the "Press START to reset" again, I press
STOP.  Now it gives me the same old "paper jam press on/off" again.  I
press off.  I wait 15 seconds.  I press on again, I get the same cycle
without the 3409 error, just the "paper jam" error.  I'm hoping there
is a way to reinitiate the printer's computer.

Hi again mmyjuly,

Although I would sincerely like to help you, without having your
printer
in-hand, I think this is the best I can do for you. Following is HP's
step-by-step guide for dealing with the document jam error -
basically, it's clean, clean, clean and "If the error persists, repeat
Steps 1-10 above, using clean cloth or swabs each time, until no dirt
or ink residue appears on the cloth or swab."  Sorry I can't be of
more help.

HP All-in-One and Fax Products - Resolving "Clear Paper Jam, Then
Press START" Error Messages for the HP Officejet 500, 600, 700, and HP
Fax 910, 920, and 925 Products:
http://h20015.www2.hp.com/en/document.jhtml?lc=en&docName=bpu50320#P26_1658

Good luck,
hummer
p.s. Please excuse the poor formatting of my first post.