If you have to execute maintenance on your own Yamaha XS 400 SE then it is always recommended to own a Workshop Manual to keep just for a reference in the event you need to know how to take off a certain part for example Door and fit it again accurately.
Yamaha XS 400 SE Workshop Manuals gives you specific diagnostic wisdom for the vehicle so if something is faulty it will help you determine what the situation is and tips to resolve it. After that you can decide if you can do this by yourself or in case the job is just too big you are able to make a reservation for your XS 400 SE in with a suitable car garage.
Yamaha XS 400 SE manual