How to (maybe) fix a streaking HP LaserJet Enterprise 500 transfer unit

I have an HP LaserJet Enterprise 500 printer, specifically the m551dn (this advice also applies to the m551n and m551xh). Recently vertical streaks started appearing on many printed pages. Repeated cleaning cycles did not fix the streaking.

I contacted HP, and they said to take out the transfer unit and see if there were toner streaks visible on it in the same position as on the printouts. There were, and when I told them that they said I would have to replace the transfer unit. It costs around $400, half of what I originally paid for the printer, so I decided to look for other solutions.

I need to tell you this before you read any further: anything you do based on what I’ve written here is done at your own risk. I offer no guarantees or warranties that this advice is sound or good or useful or clever. You may make the problem worse. You may void any manufacturer warranty there might be on your printer. You may make yourself sick from breathing in toner if you’re not careful. I personally figured that I didn’t have anything to lose since if I wasn’t able to solve the problem I was going to have to spend at least $400 on a new printer or transfer unit anyway, but the math might be different for you. Don’t say I didn’t warn you.

I noticed when I took out the transfer unit to inspect it that a small amount of toner seemed to leak out of one of the ends of the unit as I moved it around, so I theorized that perhaps the streaking was being caused by accumulated toner inside the unit. I therefore set the unit on its edge over a large sheet of paper — to collect the toner — and started tapping gently with a small rubber mallet on the top corner of the unit. I did this for a long time. A lot of toner poured out of the bottom corner of the unit (the pile of toner shown in the photo isn’t nearly all of it, since I replaced the paper with a clean one several times):

I wore a paint-sanding mask (available from a hardware or home improvement store) while doing this to avoid inhaling toner.

I then put the transfer unit back into the printer and tried again. Alas, the streaks were still there. Since, again, if I wasn’t able to fix them I was going to have to replace the transfer unit anyway, I decided to take the transfer unit out again and see if there was anything else to be done.

Here’s what the transfer unit looks like on the opposite side from the photo shown above:

Upon close examination, I noticed this in the corner:

“Hmm,” I said to myself. “That looks like a tab that I might be able to pull to remove this panel from the unit, and the other corner has a similar tab.”

Indeed, a careful application of force to the two tabs allowed me to open and remove the panel:

Wow, that’s a lot of toner still left in the unit even after all the tapping I did with the mallet!

I cleaned everything out as best as I could, again while wearing a mask for safety. Note that the grey grate shown in the photo flips up to give you access to stuff on underneath it. After tapping out as much loose toner as I could, I use cotton swabs to clean all the parts near the transfer unit. I also carefully slipped a piece of paper between the transfer belt and the rubber strip underneath the grey grate and ran it the length of the strip to attempt to rub off any toner that might have been stuck on the underside of the rubber and causing the streaking. Then I put everything back together and put the transfer unit back into the printer. So far, no streaking.

If you decide to try this, here are a few precautions to keep in mind:

• If you scratch the transfer belt or touch it with your fingers or anything else oily or basically in any way mess it up, you could cause permanent print quality issues. Be careful.
• The little gears — which you can see in the bottom corner of the open unit in the photo above — can become partially dislodge or pop out while the panel is removed. I think moving the belt makes this more likely to happen because the gears turn when the built is moving, so don’t do that. If it does happen, hopefully you can snap the gears back into place, because if not, it’s time for a new transfer unit.
• Don’t forget to put the panel back on and snap it fully into place before putting the transfer unit back into the printer.
• Wear a mask when there’s toner in the air!
• You might see other people writing online about this issue suggest using compressed air or an air gun to blow toner out of the unit. I suppose this might be a reasonable idea if you do it outside and wear a mask, but generally speaking, it is not a good idea to blow printer toner all over the place; that stuff is just not good for you. Carefully tapping toner onto paper and/or cleaning with cotton swabs or soft cloths seems like a much safer idea to me.
• Remember, you’re just cleaning the parts surrounding the belt itself. Don’t try to clean the belt itself. Loose toner will be cleaned off the belt automaticallly by the unit if it’s working properly; if not, then your unit probably really does need to be replaced. Trying to clean the belt itself is likely to scratch or damage it.
• Don’t blame me if something goes wrong. I’m just telling you what worked for me, not saying it’s a good idea for you.

Comment below if you found this useful!

(Cross-posted to the HP Community.)