Citizen CT-S4000 Service Manual page 13

Black Mark detection error (in Black Mark mode)
When Black Mark cannot be detected even if a certain amount of paper feed is carried out for
Black mark detection, a Black Mark detection error occurs. If black detection continues more
than the specified period, a No Paper condition is assumed and the same error as No Paper is
indicated.
Lighting and blinking status of each error including the above is shown below.
Caution: In many cases, "Low voltage error" is caused by blown F2 fuse.Please check the
conduction of F2 fuse.
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CT-S4000 Service Manual