This may be what your looking for;
Problem:
Unable to access diagnostics, the following message appears: "Starting Diagnostics". If the icon is selected again a new message "Unable to get into diagnostics, already being accessed." or "Diagnostics has already been started." is displayed. This is often found when trying to clear a C17-350, Create critical files, at install.

Cause:
The controller may have locked up while diagnostics was being used. In severe cases, the diagnostics becomes corrupted.

Solution:
1) Perform PROCEDURE A
2) If that fails, perform PROCEDURE B
3) If that fails, perform the actions described in Eureka # 142865 (as if it were 2.x SW).
4) If that fails, it may be necessary to reload the controller software (refer to the General Procedures in service documentation).

PROCEDURE A:
============
1) Login as root user (superuser "SU/service!")
2) For 6135 running DocuSP 1.2, cd to the /opt/XRXnps/configuration subdirectory; for 61XX with DocuSP 1.3X and higher, DT65 or DP65, cd to the /opt/XRXnps/XRXsyscnt/configuration subdirectory.
3) Display the SCMPrivate.txt file by typing "more SCMPrivate.txt" at the command line prompt. The file should display a single line of text with the entry "999" (the quotation marks are not a part of the displayed text).
4) If not, edit the file using the vi editor or textedit, change the entry to 999 and save.
NOTE: The system uses the entry in the SCMPrivate.txt file to determine whether it is in diagnostics or not. A controller lockup while in diagnostics may have prevented the system from properly editing the file as it changed state from diagnostics to normal user mode. When diagnostic entry was attempted later, the system consulted the file, saw an entry indicating it was already in diagnostics and generated the error message.)
5) Next type "ps -ea | grep dC" and press return. If diagnostics is running, the system will display a line of text resembling the following:
943 console 0:02 dcDialog
In this example, 943 is the Process ID (PID) number.
6) To stop dCDialog, enter "kill [PID]"; in this example, you would enter:
"kill 943". DocuSP should restart -- if it does not, restart the Xerox software manually from the Print Services window or reboot the system.

PROCEDURE B:
============
Delete then re-install diagnostics. For specific directions on removing diagnostics, see the ETI document entitled "How To Remove Diagnostics"; for directions on reloading diagnostics, see "Installing Diagnostics Or Gateway Software Only"

The kill comand worked for me on 4.2 when it said diag already started