The problem is usually with the sensor unit, and not with the display. I don't have a service manual for your ski, but I do have one for the GP760, and it doesn't show how to test the sensor. Best bet is to swap it out with a known good one.
If you want to try to test it: On other skis you apply 12v to the Black/Yellow and Red/Yellow leads then measure the voltage output between the Yellow and Black/Yellow leads when you spin the wheel. On your ski the wires may be Red/White, Yellow, and Black. You want to see pulses in the voltage as the wheel turns. Its actually sort of hard to test them without an oscilloscope, unless you have a very accurate multimeter.
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2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100
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