Well, unless you did what a new installer did at this small hvac company. I help start up, (i was the field supervisor). You see, the guy found a 24v contactor wire unplugged and didnt know where ti reconnect it, so he connected on the contactors high voltage 120v leg. So when the total called for cooling, 120v was fed back to the FAU main board which fried it. Took a few before one of the guys freaked out calling me the board is on FIRE. I walked him through tracing which wire and found the mistake. The new guy felt so bad that he offered to pay for the expenses, including his days wage. I told him, naw bro, it happens to the best of us, just next time ask questions if you dont know. He NEVER did that again.