I was given an old HP laser printer the other day a laserjet III.
I did a test page and dumps ton of toner on paper, and don't fuse it. I'm guessing bad connection or fuser shot. I popped open the toner to look at drum and drum is yellow with like black lines wrapping around it. I don't appear to be cleaning properly on the drum. think cleaning is in the toner so no way to fix. its starting to look like need new fuser and toner at least. doubting though be worth investment in something this old, and new lasers are like desktop sized now and like 60-70$. expect toner and fuser be about that price.

 

You can buy a brand new HPLJ P1102 for $80 in the US. Not worth buying a toner cartridge for an ancient printer.