A DIY owner and a two-man shop share one problem: the ski is on the trailer, the customer wants it back, and guessing costs parts. A Bluetooth dongle with a phone app handles the first hour of work without a laptop. It works on Yamaha four-strokes from 2002 on, across the FX, VX, GP, EX and FZ series plus SuperJet and JetBlaster, 37 models in all, and only four-strokes. What follows is an account of where it saves time and where it doesn't.
What you get in the first hour
The M-JET Diagnostic app reads stored fault codes in plain text and gives a possible cause and test steps with each. It clears the memory afterward. It shows up to 34 live values depending on the model: rpm, engine and intake air temperatures, intake pressure, air pressure, battery voltage, ignition timing, injection time, throttle angle, the paired TPS and APS signals, switch and relay status and, on some skis, oxygen sensors. You can log a test ride and read it back at the bench.
That is a lot of information before a single bolt comes out. Many no-start and rough-running complaints end at this stage: a battery that collapses under load, a switch stuck in the wrong state, a throttle sensor that reports nonsense.
Testing parts one at a time
The component tests are what a small shop will use most. Coils and injectors are driven one cylinder at a time, plus the fuel pump, the bypass or stepper motor where the ski has one and the YPVS where present. A three-cylinder ski gives seven components, a four-cylinder one nine. Compare cylinders with each other: three injectors that sound alike and a fourth that doesn't make a better diagnosis than any single reading. On the FX140 two coils serve four cylinders, paired 1 with 4 and 2 with 3, so one weak coil affects two cylinders at once.
Two limits are worth remembering. The pump test shows the pump runs, not how much fuel it moves. And any work near the fuel system needs good ventilation and no ignition sources.
Learning and service functions
After cleaning a throttle body or changing the battery, the idle and throttle relearn can be done from the app. RiDE models have the SCU initial learning. For service you can reset the oil change interval and maintenance warning where the ski supports it, and read total hours plus the hours per rpm band. None of that replaces a repair, and a relearn won't make up for a leak or a dirty part that is still there.
Where the mechanic takes over
A scan doesn't see worn impellers, wear rings, cavitation, clogged cooling passages, foreign matter in the intake, lost compression, a worn timing chain on the 2014 and 2015 SVHO or seized spark plug threads. It doesn't see an overfilled oil level either, which owners of TR-1 skis report as a source of lost rpm. All of that needs inspection and measurement by hand. In one reported case an EX of 2020 stalled after about 15 seconds at idle with no codes and normal live values. That shows that a clean scan is a finding, not an answer.
Several skis in the shop
The app comes in variants for 1, 3, 5 or an unlimited number of vehicles, and it recognises each model on connection, so changing customer skis doesn't mean changing settings. The pages on diagnostics for the workshop and several vehicles go into the details, and the app is available in six languages.
A workable sequence on any customer ski is voltage, stored codes, live values at idle, component tests, and only after that tools and wrenches.
