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I do not know what the root cause of your error is. With that said, it is implying that it is running the alternator at full field which will drive the charge voltage up very high and damage your batteries. I would not operate the boat with this condition present.

I also will say that Charles is out of the marine electrical market and this regulator was not their best effort to start. I'd pull it out, throw it in the trash, and replace it with two individual alternator regulators. You could yank the alternator and bring it to a shop and have it tested to confirm it is not the alternator that is defective, but I struggle to think of how the alternator could cause this error.

You also could just go to more typical, internally regulated alternators. Depending on how you use your boat (how much you ask of the alternators), this could be the least expensive, best option. You could likely get good money for the alternators that are on there now, making the swap out not very expensive.