Panasonic KX-FLB852CX Service Manual page 247

7.12.18.1. TONER DETECTION FLOW
CAUTION:
1. Toner low can be judged by continuous 5-times TONER LOW signal at only printing.
(It is not executed at.)
2. Toner full can be judged by continuous 3-times TONER FULL signal at initialization.
(It is not executed at printing.)
3. In the ordinal operation, "CHECK DRUM" is displayed when TONER EMPTY sensor does not generate a signal for 2.73
seconds.
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KX-FLB852CX / KX-FLB852CXS