Every time I have had that code it turned out to be the drum potential sensor.
Swap the yellow drum potential sensor(both sensor and board) with another color's. You mentioned that you cleaned the machine with compressed air. Did you clean the drum potential sensors? I have blown a couple of them, before switching to a small blower brush. Gentle puff. No more blown sensors.
Hope this helps.
SC28-01, SC28-02, SC28-03 or SC28-04 code.
Solution
CAUSE: Abnormal charge. Related codes to colorsare as follows:
28-01 = yellow
28-02 = magenta
28-03 = cyan
hub PRO C500 Warm-up, wrong display, incorrect operation, lock-up
28-04 = black
SOLUTION: In order, perform the following:
Switch Charge Corona with one of another color.
Change indicated color Charge Corona wire.
Switch Drum Potential Sensor (p/n 65AA-2282) with one of another color.
If the problem persists, replace HV-1 (p/n 65AA 8401 0).
Note : The Parts Catalog (page 99, REF NO. 9) lists the Drum Potential Sensor as the HV detecting sensor assembly. The assembly comes with the board,
rail, sensor and wiring.
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