Older Generac 14KW 1800rpm with Fiat 1.6L - Overspeed shutdown - new stepper motor

Stepper motor was replaced Jan 2023. Recently unit stopped working. Starts fine but increases frequency output limit of 72hz and shuts down with solid red light. I suspected stepper motor failure so replaced it today.

I replaced the stepper motor because I could not observe it moving during startup. The throttle is already at zero though and the unit cranks and runs like that. But frequency output rapidly rises past 65hz and eventually triggers an overspeed shutdown when it reaches 72hz. I don't suspect I have a bad stepper motor because I had one replaced two years ago and I just replaced that one today with brand new. Both cases, the stepper motor does not appear to move at startup but I'm guessing that's normal? The unit is already at zero throttle and is outputting over the nominal frequency output of 61-63 no load output without any throttle. Then it climbs past 65hz output on the meter and ultimately shuts down after less than a minute of total runtime (overspeed shutoff is internally set to 72hz) Any help much appreciated.

What to check?