I only had my ONAN do an overheat shut down one time.. Trust me when I say it was justified
I use a Gen-Turi when parked and only one time has it ever been so hot that I had to wear gloves to re-bag it.. (Same time) and the generator was OFF for some time
ONAN (and one assumes others) send ERROR codes obd-1 style (But easier) I don't know how many of you have ever read OBD-1 error codes but here is how (part of it)
You can either use a code reader key or the famous bent paper clip to short two terminals on the OBD port. then turn switch to on and count the flashes of the check engine light.
On the ONAN generators you either tap or long press (Forget which) the stop button and then read the flashes of the indicator light
Then you go to your handy dandy Manual.pef and look up the error code (you did download a copy of the manual to your hard drive and save a backup to your external drive right :) )
if it's overheat it will tell you. Same for out of fuel. low oil and several others though the error code can be Cryptic.