Early last year, I changed the oil, filter, air filter, fuel filter, and antifreeze on our Onan 8000QD. Then late last year, the generator would quit and show the same code, 33. I did the same thing you did, but that did not solve the problem. So, I opened the radiator fill cap, which is under a plate, and realized that the radiator was low on fluid. So I added fluid until full, and restarted the generator. It ran like a top. I learned that the radiator needed to be "burped" after changing the antifreeze.
Be sure to check the fluid in the radiator itself along with the fluid in the overflow tank.
May your smiles be many and the miles be plenty.
Karen & Allen Van Zandt