epson col stylus 680 red light always on

this printer is driving me nuts.
the red light is always on and is thus in 'lockout' mode and won't print.
From the support pdf I obtained this has two meanings:
1) printer cartidges empty
2) machine out of paper.

the cartidges are both new, which leaves me to suspect the paper...
which brings me to the question: how does the epson stylus color 680 sense
paper present? I've looked hard at where the paper rests in the tray but I
can't see anything that looks as though it would sense the presence of
paper. Does it use a mechanical arm? an optical mechanism?...
Thanks for any help.

It's not the paper, it's the ink cartridges. Even if both cartridges are
new, it seems that one of them is fault.

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