Oki C3200N Printer Sensor Error

My Oki C3200N keeps giving me a sensor error on the Cyan Drum. Panel states Sensor error C Cartridge. Any ideas what this could be. I have cleaned the printer very well and have switched the Drum and Cartridge out with another used one and still get the error. I did notice that the contacts on the drum were sort of black and I cleaned them but still have the error.

Thanks for your suggestions.

Billy Correll