My 400 Express has two push style circuit breakers on the electrical panel for the CO detectors. You may want to check your electrical panel and see if you have the same. One of them could be tripped or defective.
Also, start from the basics if you have not already done so, remove the unit from the wall and check for power at the wires coming into the unit.

 

A friend had the same problem on a 2003 480DB. We could not find the damn things for the life us, despite them being noted on the electrical diagram. So we finally broke down and called Sea Ray Customer Service. They kindly told us where the fuses were located. In this case, it was a series of glass fuses in in-line fuse holders wrapped in a bundle behind the main distribution panel. We never would have found them without the assistance of the factory.

 

Thank you very much for posting the location of the CO monitor fuses on a 480DB. You saved me a lot of time!

 

I believe ours are behind the distribution panel as well. Tons of room back there and that seems to be where they put any wiring items that need access. Let us know if you find them there!

 

Me too

 

FYI, they wear out after 5-7 years and need replacing. Mine went this past bad this week, kept going off.

 

My '05 420 ones are going off constantly! Thanks
C

 

I replaced mine with new ones and had new batteries. They still went off. Even when no engines were running.