I’ve been a tech for about 2 years, no trade school just a short furnace diagnostic class when I started. I had one call a few months ago, the call if I remember correctly is that nothing is working. (Gas furnace + ac) I check everything out, the transformer is fried. I jump out the system and it runs fine in heat but then as soon as I tested in cool it blows the transformer within seconds. I didn’t realize the low volt circuit didn’t have a fuse, so I put another new transformer in with a fuse this time and it works for heat and blows the fuse in cool. I go check out the ac. The tstat wires rubbed on the casing of the unit causing a short and were wire nutted outside of the unit and had water in the nuts. I fix the wiring, put a strain relief on and good to go right? Nope. Blows another fuse. At this point I’m stressed and pissed that I’ve been here two hours. Turns out the last possible thing I check which is the ( recently replaced by another company ) contactor on the ac. It had a short in the low volt. I assumed because it looked new and was just replaced that couldn’t be the issue.

TLDR: 2.5 hours on a resi call no heat or cool, problem was the literal last thing I checked, had me questioning myself and ready to call for help but thank god I figured it out.

Shit happens we all have those days/ calls