Repeated but fading images often called "ghosting" are seen when the the fuser is too hot and melting the toner onto the hot-roller on the fuser or indeed any of the rollers involved in the print process.
The CTD (Colour Toner Density) is a Light Sensor either on the Transfer Belt Assembly or on the Printer facing the Transfer Belt. The sensor is found in a different place depending on the Printer Model. Further information can be found in your printer's User Guide. 1.