What do the lights on the control panel mean?
A solid blue wireless status light indicates a wireless connection has been established and you can print, while a blinking wireless light means wireless is on but the printer is not connected to a network. The Attention light indicates the printer is out of paper, there is a paper jam, or another event requiring attention. The Cartridge lights indicate low ink or an ink cartridge problem. (Page 2-3)
From the official HP Deskjet 2540 All-in-One Printer manual, page 2
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