Mike,
Digging a little deeper, I have copied and pasted the manual's directions for troubleshooting the "21" series error code:
System Error: 21:YZ
Problem Description: Fail moving Service Station.
Corrective Action: Try the following:
Remove the Right Cover and make sure the cables from the Printmech
PCA to the Service Station are connected and are not damaged.
Make sure that the Service Station path is clear. Remove any visible
obstacles (e.g. screws, plastic parts, etc...) restricting the movement of the
Service Station.
Perform the Service Station diagnostic test to troubleshoot the problem
further ⇒ Page 3-45.
If the System Error continues, replace the Service Station ⇒ Page 6-60.
If the System Error continues, replace the PrintMech PCA ⇒ Page 6-71.
You can download the service manual for the Z3100-Z3200-Z3200PS for free by going to
LPS computer site and scrolling all the way to the bottom and clicking where it says
"Download service manual here" in a very small font. This is a different manual than the one that is on the Z3100 Wiki site because it covers the entire Z3100-Z3200-Z3200PS series.
Please note that I'm not in any way affiliated with LPS, but they have a series of repair videos on Youtube that I found useful to help replace my Z3200's power supply unit (PSU) recently. But I did not purchase my replacement PSU from them. And the free Z service manual is, of course, fantastic.
