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The green LED indicates that the NUC is receiving standby power from the power supply. This is a mere trickle of power used for system maintenance activities. When you press the power button to power on the NUC, it then begins drawing more power from the power supply. As that point, the main Power LED should turn on. Typically, if there is something wrong with the power supply (namely that it cannot provide the full wattage being requested by the NUC), what you will see is the system start to power on and then immediately power back off.
Bottom line, I believe that your power supply has a problem and you need to try replacing it in order to determine if this is the case.
You can purchase them pretty cheaply. Look for one that outputs 19V at a minimum of 3.42A (65W); they are even available through Amazon; here's an example: Amazon.com: HQRP AC Power Adapter for Intel NUC).
If you don't want to purchase one unnecessarily and your NUC is still under warranty (check here: Determine if Your Product Is Still Under Warranty), you can contact Intel regarding a replacement via RMA. Here is contact information, by geography, for setting this up:
You could also try their chat service (here: Intel Customer Support Chat Service) but this is subject to PST TZ hours.
Hope this helps,
Thanks for the detailed reply! I just tried a different brand new PSU (19v, 120W max) as per your suggestion and unfortunately get the same half second of life, then off. I fear this may mean something more fatal has occurred
When you press the power button, does the Power LED flash or stay on solid (until the system shuts off)?
I think it’s a flash, as it’s only on for less than a second, and the fan looks to stay on slightly longer, but it could just look that way as the fan takes a short time to spin down after losing power.
Does it stay on longer if you remove the (amber) BIOS Configuration jumper?
No, unfortunately it makes no difference to the length of the flash
Ok, I have exhausted all of the normal diagnosis steps for this kind of issue. Something must have failed on the board. If you are still under warranty (check here: Determine if Your Product Is Still Under Warranty), I suggest that you contact Intel Customer Support directly and arrange for the unit to be replaced via RMA. Here is contact information, by geography:
You can also use their chat service (here: Intel Customer Support Chat Service), but this is English-only and subject to office hours for the PST timezone.