For the HP 940c, there is only one light to indicate a possible low ink cartridge. The LED above the cartridge symbol (the only one without a button) will flash. (Note: this LED is the second from the left.) When you see the error code, open the printer's top cover. If the error is a low ink issue, the LED will stop blinking.
With the cover open, the cartridge carrier will move until the yellow arrow lines up with an icon on the printer itself. These five icons are in order from left to right (as you look at the cartridges) : tri-color ink cartridge problem, black ink cartridge problem, tri-color ink low, black ink low and cartridges OK. If both cartridges are low, replace the one indicated by the arrow. Then the cartridge carrier will move to indicate the other cartridge is low. (If only one has a problem, after you replace it, the carrier will move to the cartridge OK symbol.)
If HP offers the full-feature software for your OS, then install the software. Use the HP Deskjet Toolbox to get exactly which cartridge needs to be replaced without needing to be ready to replace both (or to monitor your ink level (approximate ink level)).
The manual for the Deskjet 940c is available as a PDF from HP: http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=en&cc=us&docIndexId=64179&taskId=125&prodTypeId=18972&prodSeriesId=35985
I hope this helps.