Replace the drum unit. The tab on the rear of the unit is possibly broken. Part # 6837A004AA
It is caused by not turning the toner cartridge lever all the way to the locked position.
Freckled legs, January 2012
A plastic switch actuator for the laser unit located on the inner left of the drum unit is broken
Win, December 2011
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I can answer this question...
My CanonIR1600 is showing sytem error E100 what can i do?