HowTo? Reset false negative paper jam on Canon MX850 All-in-one?

Summary:

My out-of-warranty Canon MX850 All-in-one is locked up with a bogus report of Error: Paper Jam. I haven’t figured out how to reset the error. Is this possible to do this?

More info:
I think the following is the sequence of events, but I am not completely sure:

• I had plain paper in the cassette lower tray, and no paper in the rear tray.

• The “Feed Switch” was set to “Rear tray”.

• The MX850 wasn’t connected to a computer, but was in standalone mode.

• I used the front panel LCD to have a nozzle check done.

• The MX850 complained about no paper in the rear tray. I pressed the “Feed Switch” to change it to “Lower Cassette tray”.

• Shortly afterwards, I got the error message on the LCD:

Error
Rear Cover
The paper is jammed.
Clear the paper and
press OK.

• There was no error code, just the above text.

• At this point, the MX850 was locked up. I fiddled with it for over an hour trying everything I could think off (turning on and off, power cord removed, tanks and print-head out, etc). It always comes back to the above Error message. When I press “OK”, it goes to “Processing” for about 30 seconds, and then beeps and returns to the Error message. I’d guess I’ve seen that error show up 20+ times. Infuriating.

• I don’t think it ever attempted to get paper from the cassette, but instead has managed to get into some “state of insanity” from which I have been unable to clear.

• I have looked throughout the printer and found no paper. I believe I followed all the instructions in the MX850 series On-screen Manual for Paper Jams. This includes setting the printer on its side, removing the cassette, and looking for paper.

• My speculation is that the sequence of things I did put it into a “state of insanity”. It is also possible that some paper jam sensor is jammed, or thinks it is jammed.

• I sent a question to Canon tech support to ask how to resolve, and their response was “needs service”.

Canon > Regrettably, it appears the unit will need to be serviced.

If the unit was purchased within a one-year period, you qualify for a Warranty Exchange ..... otherwise, take it to an Authorized Service Facility.

• I was able to get the MX850 to Service Mode, and then with some trial-and-error, got it to Idle mode. The Alarm light is on. When I press the Reset button, it Processes for about 30 seconds, and then the Alarm light comes back on.

• I am unable to get the Feed Switch to "rear tray". It is stuck on "lower cassette tray".

• On an older ip4000, I believe there is a way to get the printer reset to "from factory" settings. Is there a way to do this on the MX850?

• If I have a sheet of plain paper in the rear tray in "Service Mode" and press the Reset button, it will pull that sheet through the printer. This doesn't happen if I have a sheet of paper in the bottom cassette.

Thanks