hi guys.after the machine adds toner toner it got a black background in copy it means it is over toner then i made a few black copy and the over toner dis apear now the copy is ok after 6 or 7 copies made again the machine adds toner.and again it became over tone again any body can help me how to stop the machine from adding toner.please.iv'e change the dev unit and toner box at the back of the machine i claen the sensor on the drum munit facing the drum surface.i dont know what to do next.please

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