Assume nothing!
Especially when some other "expert" has been working on it. 1st order of business is check the easy stuff first.
I got a good example from in my 20's of being a service electrician. Had a service call because a bathroom strip light with 8 bulbs in it was not working. I first figured the switch was bad, so took switch apart and checked to see if it was working. Switich was working correctly. I then went to remove fixture thinking that the wiring to the fixture had a problem, that checked out good. I then removed a bulb and check a socket, and that checked out good. I then removed the other 7 bulbs and checked sockets, they were all good. Turns out that the bulbs were burning out one by one. Once the last one had burned out the homeowner figured something had to be wrong because all 8 bulbs could not burn out at the same time. After that experience I always check to make sure I got a good bulb first.
Also check last thing worked on 1st or 2nd depends on how close it is to what's not working. :)
see ya
ken
In my line of work, we loved nothing better than kicking a door in. Sometimes a young firefighter had to be reminded to first make sure the door was, in fact, locked.
Ergo Gateways first question back in the day on a service call in...is it plugged in?
I usually find the most difficult reason for failure and when I beat that to death, one of you fine knowledgeable folks tell me to flip a switch or something. Problem solved. 🙌
Here's a trade secret,Mechanicing is about 85% common sense assuming you can use the tools properly.