HPLJ Pro M402dn - sporadic duplexer paper jams

[Guido257]

Only *once* in a period of about 8 - 36 hours, *only* on the first duplex/two-sided print, and *only* of the first sheet of paper, there is a jam. The first sheet of paper is printing the backside. As it exits the fuser and travels up the printer back interior, the leading edge of the paper finds its path blocked at the very top of the box where the duplex flapper meets the upper output roller. Movement continues until a sensor detects the obstruction to stop the motor. Opening the rear door shows a perfectly wrinkled pattern (washboard) paper sheet which can be gently pulled from some rollers associated with the fuser area. Power off, then on, restart the duplex job. Everything functions as expected! And there are never any printing problems during that day even when the machine may be off for hours at a time. The Service Page Event Log subsection printed from the machine menu shows many entries of Code 13.0916 (Paper jam). With one-sided printing there has *never* been a jam. In virtually all other modes and possible configurations, it is, and always has been, free of any flaws. If I run a 120#  8.5 in x 11 index card thru in a dummy 2-sided mode, it will jam with half the card in the output. Pull it out. Then start 2-sided jobs as usual. All will hum perfectly for hours on end... Somehow, many hours of disuse cause the duplex flapper to temporarily seize / freeze preventing proper paper motion. How can this be fixed?

Model: HP LaserJet M402dn
Installed: February 2016
Total impressions: ~2000

2022 September 14

[Guido257]

Open rear door. There is at the top a gate which pivots horizontally a few degrees. It directs a sheet of paper either into the output bin or back thru the main cartridge to print a second side. It appears to stick ever so slightly against a rubber bumper on the left side when pulled out. Before a print job, I align the gate so that it hangs vertically away from any travel limits. I have only tried this about a dozen times, but, thus far, no jams.