My Mac shuts down and sulks for an hour
My 27–inch iMac (Late 2013) has started shutting down out of the blue. I have to disconnect it from mains power for up to an hour before it’ll start up again, as its power button stops working. Having it tested at an Apple Store didn’t find anything wrong. Can you suggest a cause?
If you don’t have an uninterruptible power supply (UPS), abrupt shutdowns can occur during mains power brownouts. However, those shouldn’t leave the power button disabled for as long as you’re encountering: that’s more likely to be a failing power supply or System Management Controller (SMC) in the Mac itself.
That should be detected by running Apple Diagnostics ( bit.ly/mfappldiag), but in the early stages may only occur when your iMac gets hot. Giving it a good clean, particularly in its cooling vents, should keep it cooler and may stave off the symptom.
You can also try resetting the SMC ( bit.ly/mfrstsmc). For desktop models apart from the iMac Pro, shut your Mac down, and disconnect its mains power cable for at least 15 seconds, then reconnect the cable and leave it alone for another five or so seconds.
If the problem continues to persist, arrange to take the iMac to a Genius Bar or an authorized service provider and have them run their more advanced diagnostics. A new power supply shouldn’t be too expensive, but a faulty SMC would require replacement of the logic board, which is much more costly.