Thats when you trace the error signal and pin it to the proper logic state.

Most error return signals are active-high, so you hold it low. When it goes high, it signals the CPU to error out.

Umm, not sure if its labeled or not, but you may be able to find a datasheet to the IC that runs the inverter. Then from there, you could follow the error return signal. Thats what I normally do if the plug pins arent labeled what they are. Most modern inverters have 3 signals: Enable, PWM, Error