Diag nosing other hardware problems

Hold d while starting up your Mac to open Apple Diagnostics, or Alt+ d to open it over the internet if it can’t be found on your Mac’s storage. (If your Mac is from prior to June 2013, you’ll need to use the Apple Hardware Test instead; see apple.co/2je4tN4 for details).

All being well, you should see the code ADP000, which means no problems have been found. Anything else suggests an error, which will either give you a starting point for your own investigation, or guidance when talking to a professional engineer. Apple documents the meanings of diagnostic codes at apple. co/2j4AGey.