HP C4780 printer will not copy; it scans and prints OK. It appears to be a software issue (Scanner and printer both work).

Seems like you have installed only printer driver. Download the scanner driver for device seperately and install the scanner software also. And try again.

Try to reset printer with hp software reset utility

Uninstall the driver for your HP C4780 and reinstall it using this page.
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?product=3794615&lc=en&cc=ph&dlc=en&lang=en&cc=ph

If you are not familiar in doing so, you can call HP on this number: 800-474-6836 (United States)

While the print function will still work, the copy function won't work if ink is low.

this is the Correct link to download the Software by choosing the Operating system but if you say if you are not able to make copies, is it from the printer itself..

what OS are you running on? win XP ? Vista? or win 7 ? or Mac?

Check for print if printer is working ok , try the copy option and if its works ok need to check for software and if copy is not working then Scanner may have dust on the glass and reflection roller clean it’s from the scanner area where you find lamp is glowing to scan paper

pls do check the message on panel which could give better information for problem

Form your issue that you are able to print and copy but unable to scan it is very clear that it is a software issue. Copy and scan fuctions work this way.

Copy - completely related to hardware.
Scan - related to software

uninstall and reinstall the full version software from Hp for the operation system of your computer.

I have the same machine and I can not fax or scan. Do I need special software?

PSC 1401 will print but it will not scan