sends the temperature report to the ECM to adjust the fuel mixture to the ambient temp setting for the engine temp
also reports the engine temp to the ECM as to when to turn on the cooling fan
you can order one from any yamaha dealer or check e bay https://www.google.com/search?q=84+RZ350+service+manual&rlz=1C1OKWM_enUS816US816&sxsrf=ALeKk00U1zLmo85bQebLCpFSyV8qSc0pKg%3A1620136882094&ei=slORYI-hBZKk_QaB35mIDQ&oq=84+RZ350+service+manual&gs_lcp=Cgdnd3Mtd2l6EAM6BwgAEEcQsANQpWhYhWpg9W5oAXACeACAAUaIAcUBkgEBM5gBAKABAaoBB2d3cy13aXrIAQjAAQE&sclient=gws-wiz&ved=0ahUKEwiPwuCamLDwAhUSUt8KHYFvBtEQ4dUDCA8&uact=5
You should hear a low whirl from the fuel pump when the key is turned to run, do you hear that? The fuse would be under the seat or under one of the side covers. Your owner's manual is the best source of location info of fuse.
please be more precise in your question , as in any particular model of yamaha ? or motorcycles in general , and common problems as in difficulties in riding the bike or mechanical wise ? If you are asking about operating a motorcycle then that will apply to Yamaha motorcycles as well .