I've never used OBD equipment on a marine engine but I diagnose vehicle engine problems everyday. On a car P0174 would point to lean mixture on bank2, on this boat engine the 0174 code also seems to point to lean mixture on bank2. The way it detects mixture is using an O2 sensor, the boat engine seems to have O2 sensors because the OBD information includes readings for mixture.
If the O2 sensor on bank2 were bad that could maybe account for the engine going into limp mode? In both 'OBD snapshot' tables posted by
@Biz603 in post #24 closed loop fuelling is disabled and the 'readings' from the O2 sensors seem unlikely to be correct because they point to an absolutely correct air fuel ratio. In practice the air fuel ratio usually flutters around 1 (where 1 is correct, above 1 usually lean, below 1 usually rich) so we might expect to see numbers like 0.997, 1.002 etc. At high engine load the engine normally gets a rich mixture so the reading is usually below 1, on a car it might go to 0.9 (you could read that as 10% rich). A lot of OBD systems on cars will create fake numbers for components they have deemed to be not working properly, they might give an A/F reading of 1 pointing to absolutely correct mixture for a failed O2 sensor.
Summing up, I'm suggesting the O2 / lambda sensor on cylinder bank 2 could be at fault...
