Emissions control alarm....

I'm getting a port side emissions control alarm (see attached) that typically sounds at 20+ mph after about 10 minutes of cruising. Computer takes control and throttles back the engines. Each time it happens, all engine readings on VesselView look normal. We can idle slowly with no problems.

Any idea what may be causing this and how to fix?

Going to need to connect it to scan the motor, but my guess is you have an oxygen sensor issue, could also be a catalysis issue, but probability is low for catalysis

Thanks! Will contact service after the holidays.