fault codes onan generator

you say codes but what problem are you having

Code 37 is Invalid Set Configuration.

This code is a very sensitive code to troubleshoot. Some of the things you will be able to test, and some you will not.
Even the things you can test are not going to be easy.
This code really needs a special tool from Onan called a Break Out tool.
Most of the checks on this code will require you to work on some very small wires in a very tight spot to access.

I must warn you, that without the Break Out tool, you may not be able to find the problem.