If you choose to conduct auto repairs on your Yamaha XS 400 DOHC then it is always a good idea to own a Workshop Manual to use as reference in case that you want to know methods to take out a specific part much like the Gearbox and fit it again perfectly.
Yamaha XS 400 DOHC Workshop Manuals provide you with certain diagnostic info on your vehicle therefore if something is wrong it helps you discover what the problem is and some ways to correct it. You can then assess if this can be achieved yourself or that the job is too big you could make a booking for your XS 400 DOHC right into a suitable mechanic.
Yamaha XS 400 DOHC manual