This Aficio 551 shows a P1 Jam (entrance sensor) if there are no originals in the B301 ADF. LED100 on the ADF main board is constantly lit. (LED100 constant="entrance sensor".) (There's no paper to be seen anywhere near the entrance sensor.)

When originals are in the ADF, LED100 flashes. (LED100 flashing="feed-in motor abnormal.") Since the machine and ADF work fine with originals in place, I figure the feed-in motor works ok.

If originals are removed, the jam message comes back. If you put originals back in and open/shut an adf door, all is good with the world except LED100 keeps flashing.

I replaced the entrance and registration sensors, their wire harnesses, and the ADF main control board. I also tried another I/O board and an NVRAM clear. I cleaned and tested all the optical sensors. Nothing helped.

Of course, one solution is for the owner to trash the machine and get a new one. Although a solution, is not an option in this case.

Any ideas?