Troubleshooting Faded Color Issues in the HP Enterprise Pro M551

The other day I had a customer bring in an HP M551 printer that was not printing Magenta on half the page. Going left to right the color started to fade in the middle of the page so the right side of the page was completely void of Magenta. The service manual doesn’t seem to tackle this issue so I thought I would write about it.

Troubleshooting Process
First, print the Print Quality (PQ) test pages to get a clear idea of how all the toner cartridges are working.

Then disable thecartridge check option and swap the problem toner cartridge with another cartridge. For example, put the Magenta toner cartridge in the Cyan slot and the Cyan in the Magenta slot and the Print Quality test again. This will tell you if the problem follows the cartridge or stays with the slot. If it follows the cartridge then replace the toner cartridge. If you want to check the cartridge read below for things to look for on the cartridge.

  • The laser beam writes to the drum from underneath. There is a narrow opening on the bottom of the toner cartridge to allow this to happen. Turn the cartridge over and make sure nothing is covering the opening like tape or possibly a warped piece of plastic.
  • On top of the cartridge you should be able to see the drum and the developer mag roller. The drum is green in color and the mag roller has a coating of toner across the surface. Make sure the mag roller and drum are touching. The cartridge is spring loaded so pressure holds the rollers together. If one of the springs is missing or something falls down into the cartridge these rollers might not touch. If they are not touching then no toner will be applied to that part of the drum.
  • Check the mag roller. There should be a level consistent cover of toner across the surface. If this isn’t happening then either the toner cartridge is running out of toner or all the sealing tape did not get removed. On these machines I seen both happen. I’ve seen the cartridges run out of toner before the machine thought it should be and I’ve seen the toner seals get torn when people pulled them out, leaving part of it inside the toner.

If you have found that the toner cartridge is not the issue and the problem stays with the slot then follow these steps:

  • The laser scanner assembly is underneath the cartridges. When you close the door an actuator moves a shutter back to expose the laser optics. Push the actuator in and make sure that it works properly and nothing is on the glass of the laser scanner. Make sure the actuator on the door, itself, is correctly installed.
  • Remove other cartridges and compare the slots inside. Make sure everything matches from slot to slot.

What ended up helping to solve my problem on this machine was to run the Print Quality Test pages in the Diagnostic Menu. The fifth page prints a full page of Magenta. I opened the door as the fifth page started to print.

Print the PQ test pages and determine which page would help you best diagnose your problem and stop it on that page.  I used the fifth page because that’s where my problem was.  Depending on your issue you might need to stop it on a different page.

When I took the cartridge out half the image was still it and the other half was on the transfer belt. Therefore, I decided to take a closer look at the transfer belt. Visibly I could find nothing wrong. The belt was intact, the contacts looked good, and the cam assembly moved the four transfer rollers in place like it should. So, I decided to take it apart and rebuild it, clean all the contact points and swap the yellow transfer roller with the magenta. I put everything back together and the machine started printing fine. Somewhere in the transfer belt one of the contacts was failing to put a full charge on the Magenta transfer roller.

I looked up the transfer belt to see how much they cost and I was surprised at the cost. They seem to be very expensive. If you run into this problem it might not hurt to take it apart and rebuild it. If you do decide to take it apart, be careful as the belt is fragile.

Also, when I took the accumulator cleaning assembly off the unit there is a bunch of little tiny gears. Make sure you keep them together as much as possible. That was the main problem I had when putting everything back together.

Kevin Gumpp is a certified printer technician and freelance writer for Market Point. Market Point is a HP PartsOne Partner selling HP printer parts. If you have a question regarding this topic or have any other printer repair related questions or topics for which you would like more information, please send an email to

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s