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And, I should have said, once you eliminate the cooling system, the fuel system, and each of the sensors, I suggest that it's time to get an Onan tech.
I know that you probably know this, but "plenty of water coming out the exhaust" is not a measure of sufficient water, or of the colling system. The only measure of the cooling system is the temperature difference between the exhaust elbow and the engine block or oil temperature. The normal operating temperature of the block is around 80C and if the cooling system is working the exhaust elbow should be around 50C. To eliminate the cooling system as a problem, measure these temps. BTW, I cannot count how many times I have been told "plenty of water" when it was not enough water coming out.
Once the above is eliminated and the fuel system is eliminated (is the fuel pump within 8 years of age and the filters new), it is likely a sensor issue. Some model Onans have Normally Open (NO) sensors and some Onans have Normally Closed (NC) sensors. Once you determine (see below) which you have it is simple to bypass one sensor at a time by either disconnection one wire from the sensor for NO sensors, or jumping or connecting the two wires for NC sensors. Don't do this without monitoring the exhaust temperature and ensuring it does not rise above 60C.
Here is a common sensor issue (if you have the flow sensor installed). This sensor was partially clogged. Sometimes it worked and other times it shutdown the Onan. Also this sensor was found to be oriented where the debris would fall into the sensor. When it was reinstalled, it was oriented correctly (last photo):
Sensor oriented incorrectly. Sensor pipe should be turned 180 degrees:
On my first sail trip of the season, my Onan MDKAV shuts down under load after 15 to 20 minutes. No blinking codes. Does not matter is the load is from the watermaker or AC. It does not shut down from a charger load, or any load under about 15 amps. I changed all the fuel filters but that did not help. Plenty of water coming out the exhaust. Overheating should generate a shutdown code in any case.
Sabbatical III, SM419, Rhode Island, USA