HP 8550mfp Service Manual - Page 454

Note Color-plane registration (See figure 224 on page 443 for an example of this print defect.) Poor color-plane registration is characterized by "bleeding" colors, by apparent gaps in color, and by colors offset from each other or from black. The examples on page 443 show all three types:
In the upper left, yellow is offset from colors that use yellow.
In the upper right, colors are offset from black, causing a noticeable gap.
At the bottom, the double line is caused by black offset from colors. Use the troubleshooting list below to resolve poor color-plane registration. If the problem still exists after you complete step 1, try step 2. If step 2 fails to solve the problem, try step 3, and so on. 1 Check manufacturing codes on toner cartridges if the problem appears after replacing a color toner cartridge (see "Toner cartridge manufacturing codes" on page 456). 2 Check the green handle (see figure 252 on page 524, reference #6); the locking pins on the back of the handle can break or even can be sheared off if, when the transfer drum is removed, the green handle is not swung all the way to the right. 3 Check the transfer drum guide (see figure 260 on page 540, reference #40) and replace it if it has developed cracks. 4 Check engine settings 1 and 2 in service mode and compare the values to the numbers shown on the engine settings tag (see figure 79 on page 228, callout 3). Correct the settings to match the tag. The last two digits of engine setting 2 might not match; this is normal. 5 Find out if the printer has operated recently in extreme temperature conditions. 6 Replace the transfer drum. 452 Chapter 7 - Troubleshooting EN