The old adage.
If in doubt go back to basics! Still applies.
How many when en-route to a break down have, in their mind at least got the errant chiller etc stripped to almost a rebuild state?
And when you arrive and take stock it's a 5Amp control fuse! That actually happened to me once after 2 hrs of driving to site.

Or the chillers that randomly tripped out on their flow switches.
To eventually discover that the site operative that monitored the glycol levels from an opening within the buffer tank. Had previously lost the plastic cup off the end of his stick!
This had sunk and was periodically being drawn into the water flow outlet, small end first.
Compounding the circulating pumps pressure as the more they sucked the more the cup stuck.
That one took a long time to find.

One of the best is from years ago though. Where the newly qualified plumber on this site wired up a boiler temporally to a 13amp 3 pin plug and socket.
On failing to be able to prove the boiler. He calls his boss, who together with another Engineer spend all afternoon changing fan sensors, circuit boards etc.
When at the end of a long afternoon they actually check the incoming supply.

Yep the young guy had wired up the live and neutral the wrong way around in the plug.
Should of checked the basics.
Grizzly