Did you ever get a resolution to this problem? I think it may have something to do with the auger or developer. I am having the same problem.
Take a look at the bottom of the toner cartridge and if the metal tab look like in the first picture, push the little tab in the middle inside like in the second picture and after that push the entire metal tab to the center of the cartridge like in the 3rd picture. Insert the toner cartridge in the machine, close the front cover and see what is happened.
If the problem is not solved, than replace the part that what you specified.
When I reset the toner, sometimes the graphic display will then show the cyan toner as completely full, then within a few prints it shows almost empty, then it stops printing and says replace cyan toner. Anyway, is this the sensor that might be the problem? If so, I can take one out of another dead 7750. I guess I just have to figure out how to get that top piece off to get to it.
In the Developer housing there is an ATC (automatic concentration control) sensor. Otherwise it could be a problem with CRUM connector that make contact with toner cartridge chip.
The print looks fine, it is not missing the cyan toner. I suspect there is something wrong with some sensor that measures when the toner is empty.
How the print look? Is missing cyan on it? It is possible to have a problem with Cyan Developer Unit and for replacing it you need a tech. It is not quiet simple to replace it.
My Phaser 7750 has suddenly started telling me to replace the Cyan toner cartridge when there's at least 50% left in the cartridge. I thought it might be the cartridge, so I put another one in, same thing. If I pull it out and put it back in, it will sometimes (but not always) reset itself and print about 10-15 pages then quit with the same "replace Cyan toner" message again. If I need to replace a part, I have two "dead" 7750s I can take parts from, but might need a little guidance there too. Thanks.