A local shop has a diagnostics tool which is working with a 496HO and other standard Merc engines.

Is this likely to work on my -04 377 Scorpion?

Yes it will work.

Thanks, that's good to know.

Does my ECU store error data for "offline" diagnostics?

Anyone?

Do you mean with no power to it? if so mefi 3 or 4 will not to my understanding. the newer 555 i am not sure about.

call EB Marine they can give the info you need,
615-714-2944

I'm meaning without the engines up and running.
For example while winterconcervated, though with ignition on.