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What size Onan is this?
The Onan control board needs to see proper generator voltage and a 13 code means the generator voltage is too low.
Cause could be a defective voltage regulator, but before you replace it check the condition of the brushes and slip rings that couple the voltage regulator output to the spinning field coil. Look through the cooling slots at the rear of the generator - there are (2) bronze slip rings and they should be clean and shiny, and the brushes that make contact with them should also be in good shape. If they're not, that's the first thing to make right.
If you're not comfortable working around the electrical parts of the generator, let a pro do the work. Otherwise, Flight Systems has an excellent troubleshooting guide on their website, and they also make a nice slip ring cleaning tool.
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