Phaser 8400 Printing Blurry

The cause of the blurry prints is that oil, from the maintenance kit, has leaked onto the drum encoder assembly. The drum encoder is responisble for position the Y-Axis or vertical direction of the print. The oil on the encoder is making the printer incorrectly position the drum as the pinthead is spray ink onto it, thus you get a blurry print. If you look closely, letters probably have a downward jag to them.
There are two ways to fix this. The cheapest and easiest method is to remove the right side cover, then the fan on the right. Behind that fan is the drum encoder wheel. Using a lint free cloth and isopropyl alchol (preferably 99%), clean all the oil off of the sensor disc. This method might work for a while, but the problem will ussually come back.

The second method would be to just replace the drum assembly. The drum assembly comes with a new encoder disc.
The display (menu screen) can easily be fixed by replacing the control panel (part# 848K06552). It's pretty simple to replace w/ no tools required. Here is video to show you how.

What normally causes that is the Drum motor encoder picks up the lubrucant from the drum and makes the drum spin to fast and or just out of sync.
It is located on the right side of the machine. I reference that looking from the front. To remove the right cover you remove one torx screw from the top back and one inside the paper path door in front, near the top. Then when those 2 are removed you lift up and out on the cover.
The encoder is on the end of the drum shaft. You can remove it and clean it by pouring alcohol over it, wiping it gently then allow it time to dry.
That should take care of it. If not then the encoder will need replaced.
If you go to Xerox.com and click on support and drivers, enter 8400 in the search box, and it may be on the support page or the documentation page, but there is a parts list on the site that you can look at as a reference or also to get a part number to order a new one if needed. Using the parts list is the best way to be certain you are getting to the correct part and not guessing.
But I am about 90 percent certain that the encoder is the issue.

You remove the right side cover, as viewed from the front of the machine.It is located about midway back on the machine, You want to locate the shaft for the drum, It is located on the motor on the end of that shaft. Once you remove the encoder, you flush it out with alcohol and allow it to dry before replacing and running.

If you remove the right side cover, locate the drum shaft about midway back and midway up.
There is an encoder on the end of that shaft. Remove it and clean it with some alcohol. Don't be afraid to flush it a few times. Let it dry and reinstall it. It get's contaminated with the silicon oil from the maintenance kit and causes a very blurry print because the drum runs the wrong speed.
That may need replaced, but the vast majority of the time, cleaning the encoder resolves that problem.

If it is blurred, the problem is the drum motor encoder. It picks up the silicon lubricant from the maintenance kit, and once its gets contaminated it causes the drum to run the wrong speed.
What you have to do is remove the right cover, looking from the front.
Then locate the drum. Its about mid way back on the machine. The encoder is in that area.
If you go to Xerox.com, there is a parts list and diagrams located under drivers and documentation.
Look up the parts list and the Drum encoder to locate it. It is not real hard to find, or remove and clean. What you want to clean it with is alcohol. No no, Not vodka..... use a strong rubbing alcohol to break down the silicon and clean the encoder, then put it back on the machine.
It may need replaced, but the majority of the time, cleaning the encoder resolves the problem.
You may just want to write down the part number when you locate it, just in case you would have to order one. But I really believe cleaning it will resolve your problem. It does fix the blurrs about 80 percent of the time.

There is an encoder on the drum drive motor that picks up lubricant from the maintenance kit. It may simply need cleaned, if cleaning doesnt work, it will need to be replaced.
Xerox has parts lists on the xerox.com site for that product if you need to locate it.

That can be one of two things.
1. There is a drum encoder on the end of the drum motor.that picks up silicon lubricant from the maintenance kit roller. Normally removing it and cleaning it will resolve the issue, sometimes it needs replaced.
2. There is a print head block that can become dirty and not allow the print head to get into the proper printing position, but that most likely would cause an 07,10.043 code and shut down. I really suspect a dirty/contaminated encoder.
The encoder is on the right side of the machine looking from the front, once you locate the drum shaft you will see it. Sort of flush it out with alcohol and allow it to dry, put it back in and you should resolve your problem. If not let me know.
If you need to order one the part number with diagram is on Xerox.com, under downloads and documentation, there is an online parts list for that machine. www.xerox.com

There is an encoder on teh drum motor that picks up lubricant from the maintenance kit and causes those blurs. It eneeds cleaned or replaced

Most likely the drum encoder has picked up lubricant from the maintenance kit. If you remove the right side cover it is on the end of the drum shaft. Normally cleaning that well resolves blurrs.
The drum itself may not getting perfectly into print position either. A cable hanging it up just a little to far from the drum.
But most likely the encoder needs cleaned. Possibly replaced.

It sounds like the drum encoder is bad and will need to be replaced. Is it still under warranty? Call Xerox. Or find someone who is trained to fix printers to repair it for you.

Good Luck.