Onan over charging - code 12 error

[louiemontana]

This generator problem has me puzzled. The generator motor shut down, and a Code 12 "Over Voltage" was displayed. I re-started the generator, it ran for a few seconds and then shut down and again a Code 12. I measured the voltage at the customer connection block. The voltage between T-1 and T-2 was 120 until the generator kicked up, then it went up to 210. Between T-3 and T-4, the voltage was 0. The motor immediately shut down.
I think the generator saw not enough output so it compensated, then saw too much voltage on one leg and shut down. The continuity through the stater and rotor windings was good. Could this over-voltage on one leg and 0 voltage on the other be due to a defective control board? Can anyone out there give me guidance on how to confirm if it is the control board or other things that should be checked first? This generator is a 12.5 HD CAB Spec A with less than 500 hours on it.
Any advice is appreciated.
Louie Montana