Hi Kuau,
I too have a HP Z3100PS printer, and I had mine serviced a while ago as a last resort. This ran kinda long, but this is a true story.
I had done the "belt job replacement," with the help of the forum info and videos online, and although I replaced the belt and got the machine back together just fine, something else prevented success.
I had run out of time so I quickly bought a new Z3200PS, and then it arrived damaged in shipping. HP was called out, and fixed that, and after consultation with the tech, I let HP service fix the older Z3100PS. The tech went all out, but had a terrible time tracking down the "issue," and in the end he fixed it by replacing a long piece of thin wire (reminded me of simple "bell wire") that ran the length of the back of the printer that got it working again. If anyone knows what that wire is called, please let me know.
Before he replaced that wire, he worked on the printer for weeks replacing part after part, and he replaced so many parts! Fedex brought box after box after box of parts, and when it finally came back to life I was relieved.
Fast forward to just the other day. I fired it up. The Z3100PS had been turned off while I was away from the studio for about a month. When I started it up expecting things to be fine as they have been for a long time...
I got an error, I looked it up in the manual and did what it recommended: pull the plug out, put it back in and restart. So I did, and it restarted, and this time I got a 2nd different error, so I looked it up and the same remedy was recommended and I did that again, and when I restarted it I got yet another 3rd different error, and the same recommended pull the plug out, put it back in and restart was followed yet again. The reboot for the 4th time came up totally OK. I printed out a diagnostic imagine, and all is spot on OK.
Pretty strange? Or not?
I wish I had a real answer for you, but I tell you of my experience to urge you to keep on going.
So long for now, Tom