Swap for known-good parts

A standard technique for fault finding, but one which requires care.

Eliminate possible causes by swapping parts of the system for known-working replacement parts, ideally taken from a supposedly-identical system which is working correctly.

If the parts are expensive or you don’t have many left, be careful because a fault elsewhere in the system could destroy the component and leave you with two broken systems. I’ve had a situation where an expensive sensor was dead, and replacing it with a new one just resulted in another expensive dead sensor – a wiring error elsewhere was feeding it the wrong voltage and frying the sensors as soon as the system was powered up.