You are standing at a jetty next to a used Yamaha four-stroke, 2002 or newer, with the seller waiting. A scan doesn't take long if you go through it in a sensible order, and it gives you facts to put against the asking story. The M-JET Diagnostic runs on a phone, so you need no laptop on a wet dock, and it supports the FX, VX, GP, EX and FZ series, the SuperJet and the JetBlaster.
The order I would use
- Connect with the ski off the trailer and the engine not yet started. Read the vehicle data first: model, engine serial number, SCC code, ECM part number and software version. Compare model and serial number with what is on the ski and in the paperwork.
- Read the fault memory and write down every entry with its description. Don't clear anything. A memory that has just been cleared tells you less than one that is full, and a seller who clears it in front of you has told you something too.
- Read the total engine hours and the hours per rpm band, and put them next to the display counter. A large share in the top bands means a hard life. The page on reading operating hours explains the bands.
- With the key on and the engine off, look at the live values. Battery voltage should be healthy, the intake pressure and the air pressure should read close to each other, and on a cold ski the temperatures should sit close to the air around you.
- Run the individual component tests: every coil and every injector one at a time, plus the fuel pump. A three-cylinder ski has seven components, a four-cylinder ski nine. Fuel system tests need good ventilation and no ignition sources. The pump test only shows the pump runs, not how much it delivers.
- Start the engine on the water, never on land without a water supply. Watch idle rpm, battery voltage and temperatures as they settle.
- Take a test ride with the data logger running. Include a full throttle run and a slow stretch, then look at the recording back at the jetty.
Reading what you found
Stored codes are not a verdict. An old sporadic entry may be a connector that has since been cleaned, and an empty memory doesn't make a ski healthy. Look for patterns instead: the same entry stored more than once, or a temperature that climbs on the test ride and doesn't come back down.
Check that numbers fit the model. Codes differ between control unit generations, with the older FX140, FX1000, FX1100 and GP1300R using one numbering and the newer skis another, so go by the description in the app.
What the scan can't see
- Compression loss.
- Impeller and wear ring condition, and cavitation.
- Debris in the inlet and clogged cooling passages.
- Timing chain wear. This matters on supercharged models, see SVHO and SHO diagnostics.
- Oil level and oil in the air filter.
- Corroded connectors that never set a code.
- Spark plugs that have seized in the head.
Those need your eyes, your hands and, for the engine itself, a mechanic you trust. Use the scan to decide whether the ski deserves that inspection, and to give you something concrete to negotiate on.
If anything on the list looks wrong and the seller has an explanation, ask for it in writing or walk away. If everything is clean and the logged ride looks the way the ski should behave, you have a much better basis than a quick run around the bay.
