Asus Z8NA-D6 - Manual (Page 24)

2-4 Chapter 2: Hardware information
2. CPU warning LED (ERR_CPU1, ERR_CPU2)
The CPU warning LEDs light up to indicate that an impending failure of the
corresponding CPU.
3. BMC LED (BMC_LED1) (Z8NA-D6 model only)
The green heartbeat LED blinks per second to indicate that the ASMB4 is
working normally.
The heartbeat LED functions only when you install the ASUS ASMB4.
The warning LEDs function only when you install the ASUS ASMB4.