14.8.7 Defective Print Jobs and Data Transfer Errors
If you switch the printer o or shut down the computer during the printing process,
print jobs remain in the queue. Printing resumes when the computer (or the printer)
is switched back on. Defective print jobs must be removed from the queue with
cancel.
If a print job is defective or an error occurs in the communication between the host
and the printer, the printer prints numerous sheets of paper with unintelligible
characters, because it is unable to process the data correctly. To rectify this situation,
follow these steps:
1 To stop printing, remove all paper from the printer or open its paper trays.
High-quality printers have a button for canceling the current printout.
2 The print job may still be in the queue, because jobs are only removed after
they are sent completely to the printer. Use lpstat -o or lpstat -h
cups.example.com -o to check which queue is currently printing. Delete the
print job with cancel
queue
-
jobnumber
or cancel -h cups.example.com
queue
-
jobnumber
.
3 Some data may still be transferred to the printer even though the print job
has been deleted from the queue. Check if a CUPS back-end process is still
running for the respective queue and terminate it. For example, for a printer
connected to the parallel port, the command fuser -k /dev/lp0 can be used
to terminate all processes that are still accessing the printer (more precisely:
the parallel port).
4 Reset the printer completely by switching it o for some time. Then insert the
paper and turn on the printer.
14.8.8 Debugging the CUPS Print System
Use the following generic procedure to locate problems in the CUPS print system:
1
Set LogLevel debug in /etc/cups/cupsd.conf.
2
Stop cupsd.
3
Remove /var/log/cups/error_log* to avoid having to search through very
large log les.
4
Start cupsd.
5 Repeat the action that led to the problem.
6
Check the messages in /var/log/cups/error_log* to identify the cause of the
problem.
178 Start-Up
