Hi- the flame circuit needs to read a ground path through the flame-which is flame rectification. this is a mA circuit that, when not present whether because of a poor ground or a dirty sensor, will present itself exactly as you describe-3 tries and a lockout. this sensor usually resembles a spark igniter electrode and is at the opposite end of the burner assy. from the igniter. While the unit is on the grid, you could disconnect this sensor and see the same behavior. That being said, there must be something about the gen-sine. Perhaps a small ups/conditioner would do upstream from the control board, so it wouldn't need to be sized for the fan motor. Have you checked the frequency of genset output?