Hi, I've just fixed mine after 3 years of not having any reading at all. Maybe this will help you or someone else.
About 3 years ago mine failed completely. I had a eureka moment a couple of weeks ago and decided to go straight for a 12v issue and assume the sender was ok.  I havent found this solution anywhere else on the internet
In essence, this was the problem. Somewhere inside the instrument unit, the positive had shorted out so i have bypassed it. I know this because putting 12volts across the fuel gauge gave me a tank full reading.
here is how i did it.
1. remove side panels and screen.
2. remove instrument panel connector and undo 3 nuts.
3. undo the 7 screws on the back of the panel
4. remove the unit from the housing. DO NOT place it face down in order to avoid damage to needles.
6. Remove the 3 screws on the fuel gauge.
7. The positive terminal is the top screw- put a small piece of electrical tape over the hole in the circuit board to prevent voltage going into the pcb.
Make a hole in the electrical tape so the screw will go back in.
8. wrap a piece of exposed wire around the screw and screw the gauge back into the display.
9. make a small hole in the housing (There is a blanked off hole already. Just pierce it.
10. Feed the length of wire through.
11. reassemble the instrument display and replace it on the bike.
12. Using the right hand side side light cabling, join the new wire from the fuel gauge to the blue/white positive cable taking a 12volt feed from it.

You now need the spare key

Open the fuel tank with the spare key. Put the other key in the ignition. turn it on. You should now get a reading on the gauge which is indicative of the fuel level. This can take a few mins to settle.

Using a coat hanger or hook of some sort, lift the sender in the tank to check that the gauge climbs. Once at the very top (about 2 mins) let it down to the current fuel level and remove the hook.

Thats it, put it all back together and its job done.... 

hopefully this helps some people with the same issue i had. This is only for a break in the positive circuit and does not fault find sender unit issues.

All told it took about an hour with taking parts off and replacing, wire stripping and making bullet connectors etc...

Do not be tempted to reconnect the instrument display without it being in the housing. Too many earthed parts on the bike to cause permanent damage to the instrument unit. £440 from Yamaha for a new one and not much cheaper on the second hand market.