A 2021 Yamaha VX that dies in the middle of a ride and then starts again as if nothing happened doesn't always have a fuel or ignition problem. The stop switch on the left side of the handlebar can corrode. The symptom runs both ways: the engine shuts down for no visible reason, or it keeps running when you press the button. Either looks like a random electrical fault until somebody looks at the switch.
Which models are involved
The known case is the 2021 VX. Every Yamaha four-stroke has a stop switch and every one of them lives on a wet handlebar, so the same check makes sense on an EX, a SuperJet or an FX. I wouldn't promise that other models share the 2021 VX fault, so treat the switch as a candidate, not a verdict. The M-JET Diagnostic supports 37 Yamaha four-stroke models from 2002 on, the VX1100 and the three-cylinder VX, the EX series, SuperJet and JetBlaster among them, so the check below is available across the range.
What the switch value shows
With the ski on the trailer and the ignition on, open the live data in the M-JET Diagnostic app and find the switch status entries. Which switches are listed depends on the model, since the app shows only what the ski has. Press and release the stop switch and watch the matching value. It should change cleanly with every press and return to its rest state each time you let go.
Now wiggle the harness behind the handlebar and tap the switch housing lightly. A value that flickers while nothing is being pressed is a contact problem. A value that stays in the pressed state on its own points to a stuck or corroded switch. A value that never changes when you press means the circuit is open somewhere between the switch and the control unit.
Why the code list alone doesn't settle it
A stop switch that stops the engine is doing its job as far as the control unit is concerned, so it often leaves nothing in the fault memory. You get an empty list and still have a ski that stalled twice last weekend. That is why the live view is more useful here than the codes. A logged test ride adds to it: record the switch status together with rpm and battery voltage, ride until the stall repeats, and look at what the switch did in the second before the engine dropped.
If the switch status stays steady while the voltage dips at the moment of the cutout, the switch isn't your problem. Voltage is the next stop.
The connector first, then the switch
Corrosion tends to begin where moisture gets in, which is the plug rather than the button. Disconnect the handlebar harness connector, look for green or white deposits on the pins, clean the contacts and seat the plug firmly. Do that before buying a switch. If the trouble was a contact problem it may be gone, and if it comes back after a few rides you at least know the plug was part of the picture.
For a stop switch that doesn't stop the engine, read the status value with the engine off. Never run the engine on land without water supplied to the pump, whatever you are testing.
Where the scan stops
The app shows whether the control unit hears the switch the way it should. It can't show how much corrosion sits inside the switch housing or the cable, and a switch that behaves on the trailer can still misbehave once the ski is wet and vibrating. A clean live value rules out the part you can see in data, and the hunt goes on with the connectors, the battery and the lanyard.
Voltage values are explained under reading live data.
