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Your question is very vague. I am assuming you are having trouble with an Onan genset? Or is this fault code in the coach itself?
If it is an Onan, code 27 is Voltage Sense Lost. It means that the controller is not seeing voltage being produced by the stator, either because there is no voltage, or the controller is bad.
I'm not sure what there is to get back to me with.
If this is an Onan generator, I have already explained what the code 27 is.