Hello, The solenoid might be the problem, It could be causing the feed timing to be off when pulling the media back in for 2nd side printing. Which would cause the duplex roller not to turn on in time which would cause the according fan fold. Another issue might be with the main motor for the reversing of the media back to the duplexer. Sounds like will need a service call.
I was once pretty adept at working on laser printers, but it has been a lot of years since I had to trouble shoot one.
I have a Phaser 6115 which, despite being old, has a really low page count. I recently moved it in to my home office to get some use out of it, and it works great for office color.
However, when I try to duplex print, I get a perfect accordion fold after it goes through the duplex unit. I have scrubbed all the rubber rollers, and I can't find any broken gears within the duplex unit itself. I'm kind of not leaning toward a broken gear tooth, as the paper folds in the same spot every time.
I did recently tear it down and clean the three solenoids because they were sticking. I didn't put any tape or felt back on the solnoids (it's pretty clickey now, but it feeds paper). Could this be a solenoid problem? There is very little information about this machine on the Web, so I hope someone can assist with this.