Canon Pro-1000 series Online Manual page 749

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Cause
If printing is continued, the ink may run out.
What to Do
You are recommended to replace the ink tank.
If printing is in progress and you want to continue printing, press the printer's OK button. Then printing can
continue. In this case, if the ink runs out during printing, printing will be interrupted even if the paper is
half-printed, and the paper will be ejected.
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