The ski beeps, the revs drop by themselves, and you are a mile from the ramp trying to guess what it wants. On a number of Yamaha models, including the EX, the overheat warning and the oil pressure warning use the same tone, so the sound alone cannot tell you which problem you have. According to the EX owner's manual both warnings reduce the engine speed automatically, and the manual's advice is to clean the intake and let the engine cool down.
That advice covers the commonest case, but it does not help you decide what to do back at the dock. Here is the order I would work through.
Start with the water, because heat is the usual suspect
In forums, overheat alarms and limp mode on the EX, VX, FX SVHO and FZR are reported in large numbers, and the cause is most often a water flow problem rather than a bad sensor. Typical findings are cavitation of the pump, debris on the intake grate, a chipped impeller, a clogged small cooling-water passage, or a manifold that has silted up after poor flushing. The EX tends to show it on long rides and hardly at all on calm, short trips.
A quick check that owners like: with the engine running and water supplied, water should come out at the rear and also at the test tube at the front. Water at the rear and none at the tube means the path is blocked somewhere in between. A scan cannot see any of this, so if the water path is poor you will find it with your eyes and a mirror, not with the app.
Then read the temperatures
If the flow looks fine, open the live data and compare. The 1.8-litre models carry three temperature devices: the exhaust manifold thermo sensor (code 65), the engine temperature sensor (code 15) and a separate thermo switch. The first two show up as numbers. If neither sensor reports anything unusual and the alarm still sounds, the thermo switch becomes the suspect. Code numbers can differ by control unit generation, so go by the text in the app.
Oil pressure: read the switch, then confirm with a gauge
The oil pressure switch appears as a switch state in the live values. If it says low pressure while the engine runs, you have two possibilities: real low pressure, or a switch that is lying. On the VX110 Deluxe, owners report that the switch can rust when the rubber cap is missing or not seated, and then it triggers beeping with nothing wrong in the engine.
The app cannot tell these apart, since it only sees what the switch says. A mechanical oil pressure gauge fitted for the test tells you the truth. It does not replace a scan, but it settles this one question where nothing else will.
Battery and charging
A weak charging system can also make a ski complain. On the FX140 a corroded regulator is reported by one owner to cause alarms and starting trouble, and on the VX1100 the same part is known. Check battery voltage in the live data with the engine running: it should rise well above the resting value, and if it does not, suspect the regulator. Checking battery voltage and the charging system lists what to compare.
Where the M-JET Diagnostic fits
The Yamaha version of the M-JET Diagnostic supports 37 four-stroke models, and on all of them you can read switch status, temperatures and voltage side by side. That usually shows which alarm you are dealing with before you leave the dock. What it cannot show is a blocked water path, a worn impeller or real oil pressure, and those are the three things you check by hand next.
