From what I can tell 79.04 is an error uploading the firmware file so the above may well apply and if this is the case uploading the firmware may work.
I had no problem uploading the firmware.
printing over ethernet from my windows XP pro laptop has been no problem either from PS2 or lightroom.
but, printing from my windows 2000 workstation has been a battle. worked for a while. but now I get a 79-04 error in ps2. sending a test print from the windows 2000 printer window works fine. i have removed it from usb and am connecting via ethernet on both machines instead.
btw i found that the hp utilities when loaded from the cdrom didn't work on my machines. even with updating. when i got rid of them and then uploaded directly from the HP site and installed those they run fine.
below is an answer via email from hp. the email came very quickly.
"Thank you for contacting Hewlett-Packard's Commercial Solutions Center.
We understand that your Designjet Z3100PS displays a system error 79:04, and that cleaning the printhead and carriage contacts with a damp lint-free cloth has not corrected the issue.
Under normal circumstances, you should usually need to clean the printhead and carriage contacts only when you replace printheads or experience print quality or ink-related issues. However, ink buildup, dust paper fibers, or minor factory residue may occasionally interfere with carriage-printhead communication just enough to trigger this generic firmware error
If you regularly use highly fibrous uncoated paper types, if you regularly print borderless prints on glossy media, or if your environment is particularly dusty, you may need to clean the contacts once every few months.
Other possible causes of this error include communication, file or application issues, or driver, firmware, or hardware issues.
If the printer's power cord is plugged into a surge protector or UPS device, please plug the cord directly into a wall outlet for the duration of the troubleshooting process. Replace your surge protection device if the issue does not occur while the cord is plugged into the wall outlet.
Please disconnect the USB cable and print an internal test page from the printer's front panel. If the error occurs again, your issue may be related to firmware or hardware. If the printer test page is printed correctly, the issue is more likely to be related to communication or software.
Please use a damp lint-free cloth to wipe away any dust or ink buildup in the front of the printer, where your pages are printed and cut, if the error occurs on an internal print.
To test for communication or software issues, please reconnect the USB cable, after verifying that it is no greater than 15 feet in length. Send a Windows test page from the driver Properties, followed by files from several different software applications.
A failed test page indicates a possible communication or driver issue. Please remove and reinstall the driver, and replace the USB cable with a known-working cable from another printer or computer. On the other hand, if the error occurs when using a particular application or when printing a specific file, please check your application settings and re-save the file.
If the issue persists, we would suggest you contact HP’s Customer Care phone center in order to perform any additional troubleshooting, and if needed to determine what repair options are available for your printer. You can reach HP’s Customer Care center by calling 1-800-474-6836 (Mon-Fri, 6am to 6pm MST).
Once again, thank you for contacting Hewlett-Packard's Commercial Solutions Center.
NOTE: Our advice is strictly limited to the question(s) asked and is based on the information provided to us. Problems and solutions may depend on the nature of your system environment and various other parameters that are unknown to HP; therefore, HP cannot assume any responsibility or liability. Please be advised that technical information changes as new data becomes available, therefore, HP recommends that you check back at our technical support web site located at [a href=\"http://www.hp.com/country/us/en/support.html]http://www.hp.com/country/us/en/support.html[/url] regularly for possible updates. Hewlett-Packard shall not be liable for any direct, indirect, special, incidental or consequential damages in connection with the use of this information."