I am experiencing the same issue for the first time, where did you find the cleaning instructions?
The transfer roller unit is also a waste toner box. It has 2 set of optical sensors. When the waste toner compartment is getting full, toner powder will block the prism-shaped transparent windows hence first set of optical sensor's IR path. When waster toner compartment is completely full, waste toner powder will stick on the drum and trigger second set of optical sensor in the centre. LCD screen then displays the "Please clean CTD sensor" error message and the amber exclaimation mark lights up.
The simplest solution is to replace the transfer roller unit with a new one. Emptying the waste toner compartment and cleaning all relevant parts is not recommended as this involves health hazous of inhaling toner powder. There is also an internal mechanical flap that rotates to keep cleaning the inside of the sensor's plastic window. Even though the design is low fiction, the flap leaves permanent scratch mark on the plastic window after years long repeated contact. I just cleaned it the second time and the compartment is actually pretty empty. If the same "Please clean CTD sensor" comes back again, I have no choice but to pay for a new transfer roller (~GBP £23+).