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 …

Trace faults methodically

Sooner or later you will spend time attempting to work out why a system isn’t working. After you’ve poked it a few times and it definitely doesn’t work, start working methodically rather than just randomly changing things in the hope that you will fix it. See also you-will-forget-what-you-did-part-2/ about documenting this process.