Asus Z9NR-D12 - Manual (Page 41)

ASUS Z9NR-D12 2-21
3. CPU warning LED (ERR_CPU1/2)
The CPU warning LEDs light up to indicate that an impending failure of the
corresponding CPU.
4. CATT Error LED (CATTERR_LED1)
Indicates that the system has experienced a fatal or catastrophic error and
cannot continue to operate.
The warning LEDs function only when you install the ASUS ASMB6.