Here’s a link to the parts catalogue for a Konica-Minolta magicolor 2550DN.
There is no mention of a Toner sensor but there are 5 parts named as a photo interrupter which may be what you are seeking. Fortunately all are the same type of component as they have the same part number.
The part number is 9335130061
They are found in the following sections of the printer.
p.3 mainframe section part #7
p.14 vertical transport section part #2
p.15 fusing section part #17
p.17 paper take up section part #11
p.20 duplex unit part #10
Here’s a link to just one supplier. It is only shown to let you see if this is the part that you require and also if it is, what the cost of the part is.
If you search online using the part number only in the search term of your browser you will get results for other suppliers of the part that may suit you better
If this is not the part that you need then hopefully you will be able to find it in the parts catalogue.
You made a typo in the part number for the switch.
It is 4139 R735 00
Here’s a supplier
Not quite sure if I understood it but was the Cyan cartridge being recognized before you replaced it with a new one or not?
Usually with toner cartridges there is a chip attached which makes contact when it is inserted in position. Don’t know about your specific toner cartridge but if there is one is it unobstructed and clean?
Did you try re-installing the cartridge again as per the following?
1. Select [QUALITY MENU] → [REPLACE TONER] and the specific color of toner to be replaced.
2. The rack rotates to bring the specified color of toner cartridge to the replacement position. When the rack stops moving, the message [OPEN DOOR AND REPLACE (color) TONER] appears on the display.
3. Open the top cover and replace the toner cartridge.
4. Close the top cover. The initial screen will then reappear.
If you can’t get to the cartridge to be recognised this way here’s a link to the service manual for the printer.
To download the manual, click on the I’m not a robot checkbox below the Document preview page and then click on Go to Download.
When the sentence below the Document preview box changes from This file is downloadable free of charge (5/day): ...processing... to This file is downloadable free of charge (5/day): Download (konica minolta magicolor 2500w 2530dl 2550 dn sm) click on Download as this is the link. Be patient as it takes a little while to find the file.
On p.129 it shows how to get into the service mode for the printer
Enter the service mode and select Remove All (shown on p.130) to see if it is recognized this way when you remove and replace it..
A possibility is that it may be a faulty cartridge