HP Laserjet 2840 Mispick in the ADF, yet roller and seperator work fine

Question

We have a LaserJet 2840.
Everytime we try use the adf tray we get the error message: Mispick
We followed the manual and cleaned the rollers and seperaor - issue persisted so we replaced rollers and seperator.
We still get the "mispick" message.
Paper pulls through the adf just fine but it doesn't recognize that the paper is being pulled thoruhg.
My uneducated guess is that the "form sensor" is not working.
How can this be fixed... can this be fixed?
Any other ideas?

Answer 1

I'm having the same problem with my wife's MacBook Pro. I've fixed it three or more times. It keeps coming back, and the same fix doesn't always work. My iMac, with the exact same configuration, has not had a problem since the first time. I have a (very old now) hp 4050dn connected to our LAN via ethernet through an Airport Express configured for relay.
Console logs always have two errors, one says that post processing failed, the other says something about a filter.
Test pages and files from Text Edit work, pdfs (from Acrobat or Preview) spool, but then the printer queue stops. Print jobs just pile up in the queue, but it's never one job holding the others up.
I've deleted and recreated the printer. I've uninstalled and reinstalled drivers, including the hp 1.1 driver update (reinstalling that always worked until this last time, when nothing seems to work). I've restarted CUPS via terminal {
cups disable
cups enable
I've restarted CUPs via terminal another way {
sudo killall cupsd
sudo cupsd
I've deleted and recreated print presets. I've reset the printer in the Systems Prefs Print and Fax pane. I've repaired permissions. I've probably done a bunch of other things, too. It's come back after a week or so every time, and this time, nothing is working. Bad news.