WHY DID MY FURNACE CONTROL BOARD FAIL?

A customer who had called me had an issue where the furnace was not running at all. When I arrived at a customer’s house, I found the breaker was tripped causing no power to the furnace. I patted myself on the back and proudly proclaimed “simple fix, just flip the breaker back on”. Little did I know at the time, what was awaiting just beyond that blower door. So, after we flipped the breaker back on and it immediately tripped again, I knew we had a more serious issue. I pulled the blower door off and there it was, a terrible mess of melted wiring, connectors, and plastic. It was then that the customer remembered smelling an odd smell the night before when he turned it on for the first time. Now a rookie mistake would be buying a new control board, throwing it on, and powering it up. Good to go, right? WRONG!!! You need to remember that something caused this to happen. In my experience, control boards don’t just decide to catch on fire. So I started tracing all the wires to try and find a short somewhere. I looked for a piece of bare wire touching metal. I looked for anything that may have caused this nightmare. After coming up empty, I traced the blower wiring and noticed that the common wire connector to the capacitor was off and melted to the side of the blower box! This customer advised me that he had another company out for maintenance on the unit a couple of weeks prior but hadn’t used it since then. And he tried to have them back out but they were booked for a solid week, so he called us. It looked like the previous tech had mistakenly pulled the connector off. So I repaired all the wiring, connectors and replaced the board. I then triple-checked all my connections and powered the unit back up. The furnace was fixed and the customer was on the phone with the other company to get a refund for the work I had done. The lesson here is to always take your time when working on any appliance and triple-check all your connections!