Canon multipass c3000 Service Manual (en)

Ink empty detection
Ink empty detection during fax operation is performed by firing black ink
between the light source and receptor of the ink sensor, located on the left side
of the printer.  This function does not work with the BC-22,  only with BC-21 or
When ink passes between the source and the receptor, as shown in 
Figure 2-12
the sensor output will be a pulse waveform.  The presence of ink may be
determined from the pulse generation time.
Ink empty detection is performed at the end of each received page of printing.  If
it is determined that there is ink remaining, the image data for that page will
be erased from memory.  If the cartridge is out of ink, the message 
will be displayed to inform the user that the cartridge is out of
ink, and the page will be “received” to memory again from the image data.
The detection level of the ink detection sensor is adjusted automatically by
means of a feedback circuit, and thus requires no manual adjustment.  If the
sensor output does not reach a standard level, even after performing feedback
control (such as if the receptor is completely blocked, or if a sensor defect
occurs), the ink sensor failure will be reported with the display of the 
(error code ##348) message.
Part 1: Facsimile
Signal sends to
PCNT board
Ink detect sensor
Figure 2-12 Ink Empty Detection