Re: Yanmar coolant light on
Raw or fresh water cooled? Could be your sender is bad (not likely) or you've got a short somewhere...or worst case scenario, you're actually overheating. There may be scale in your heat exchanger, your impeller could be bad, the coolant could be gelling up in your passages.
When was the last time you flushed your coolant and/or had your exchanger cleaned/replaced?