Your F115 shows a fault number, you search for it, and the first forum thread says thermo sensor, the second says something else entirely, and a laminated chart someone handed you at a boat show says a third thing. Everyone in those threads is sure they're right. Often they are, for their own engine.
One number, more than one meaning
Yamaha outboard ECUs come in generations, each tied to the year a platform was introduced rather than the exact model year of your engine. An F115 built in a given year can carry an ECU generation that started several years earlier. The fault number lists aren't the same across those generations. In the data the app is built on, the thermo sensor sits under different numbers depending on the generation. The same number that stands for the air temperature sensor in one list stands for the knock sensor in another, and one number that means the engine shut-off switch on one ECU means variable cam timing on the starboard bank on a different one.
A chart printed for one generation will mislead you on the next. That's not anyone's mistake in the thread. It's how the systems grew.
Why some engines know a dozen faults and others over a hundred
A small F25 ECU knows somewhere between 9 and 13 fault messages. Across the F115 to F150 family it's 10 to 76 depending on generation, and the big F275 to F450 engines know between 44 and 107. The newer engines added electronic throttle, digital shift, cam timing per bank and direct injection, and each of those brought its own set of messages. A forum list put together from one engine can't cover the others, and a list that tries to cover all of them has to pick one meaning per number. That's where the confusion starts.
It gets worse with model letters. An F150 and an F150 with a different suffix letter may sit on different ECU generations, so even two owners with the same horsepower can be talking past each other.
Read the engine, not the list
What counts is the description stored for your engine's own ECU. The M-JET Diagnostic reads the messages straight from the ECU and shows each one in plain text: the component, the symptom (such as out of specification or irregular signal) and the check steps that belong to that ECU. The number becomes secondary. If someone in a forum asks which code you have, give them the component and the symptom instead. "Thermo sensor, out of specification" means the same thing on every Yamaha. A bare number doesn't.
More on how the app shows fault messages is on the page about reading fault codes.
What's still useful in forum threads
Forums are good for symptoms and causes. A post that describes what the engine did, under which conditions, and what finally fixed it is useful no matter what number the poster wrote down. Posts that only pair numbers with meanings aren't, unless you know which generation they came from.
The same caution goes for spec values. A resistance or voltage someone quotes for a sensor may apply to their engine and not to yours. Use the check steps for your own ECU and the service manual for your exact model. Then go looking for other people's experience with that component on the same engine family, and pay most attention to the posts where someone came back weeks later to say what actually fixed it.
