My Onan 5.5 was giving me a Code 58 and shutting down about 5 min after start-up. That sensor shuts the unit down at 1200 degs I replaced the Exhaust Gas Temp but heat issue continued. I must add that this unit only has 73 hrs and my Wife had complained about excessive floor heat since the unit was new! We have an Axxess 30 ft.... it was quite a job getting to the Onan to remove the cover. I removed the muffler assembly, cut the muffler open and found a clogged Cat.....so I removed the cat converter and welded the muffler back together.......my exhaust temp went down to 214 degs at the area where the cat was and 178 degs on the exit end of the muffler. No more heat issues !!
Good detective work and remedial work. I had no idea small engines ever used cat converters. I wonder what plugged it up?
I have also noticed how hot the floor and geny compartment get on mine after running it 2.5 hrs. I never ran it longer than that and never had it shut down for high temp. That's a big drop in temp after removal! Did the exhaust get louder after you removed the cat? I had to deflect the hot air coming out of the bottom of mine with thin aluminum sheet because it was blowing on the frame and outer edge wall causing it to get very hot. Now it blows straight down on the ground and reduced the heat on the floor. Your discovery sounds like a big improvement.