Why is the Asus M5A99X mainboard power LED blinking without mains attached?

Why is the Asus M5A99X mainboard power LED blinking without mains attached?

I have a M5A-99X Evo board by Asus. Recently, I found that the Power LED of the board is blinking in larger intervals (at least 1 minute dark, 1-2 seconds lit, then dark again; did not measure) even if the mains are not connected to the power supply.

The manual does not have any information on this problem. I cannot find anything in the internets, as most blinking problems refer to issues with the mains attached and usually means some overloaded bus.

Here are the datapoints I collected so far:

  • The board is rather new (less than one year), but to be sure I measured the BIOS battery (CR3032 button cell), which reads 3.03 V (open circuit) or 2.1 V (with LED attached).
  • There are no active (i.e. currently connected to the mains) externally powered USB devices attached.
  • It seems that the blinking stops while the battery is removed.
  • The system has been shut down before. No suspend-to-disk or suspend-to-ram.

Does anyone have an idea what the LED might be telling us?

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