Sporadic shutdowns
My iMac sometimes shuts down out of the blue when it’s left unattended. Apart from installing an SSD in it some time ago, it’s standard and otherwise works fine. Could there be something wrong with it? The best way to discover the reason for this is to read the shutdown cause code from the Mac’s log, shortly after you have started it up again following one of these events. This isn’t easy, but is detailed at bit.ly/mfspcd, where you’ll also see the meanings of various codes.
Before proceeding further, restart your Mac in Apple Diagnostics ( bit.ly/appldiag) or Hardware Test ( bit.ly/applhwt), depending on the age of your Mac, and ensure that there’s no underlying fault in the hardware. If you installed the SSD yourself, it’s worth checking that all the cables inside are firmly connected, too, as one may have become loose.
If the shutdown cause code is 0 and there’s no evidence of a hardware problem, the most likely reason is mains power failure, or at least a short brownout. The best protection you can afford your Mac is an uninterruptible power supply (UPS), which should ensure that it doesn’t do this again. Get one with a USB connection, so that it can signal the Mac to shut itself down if the mains power fails.
UPSes work by filtering out spikes in the mains supply, and compensating for power loss by switching over to their battery when a brownout or power loss occurs. Get the largest capacity battery that you can afford, to give your Mac plenty of warning and time to safely shut down before the UPS’s built-in battery runs out of power.