Asus P9D-MH/SAS/10G-DUAL - Manual (Page 43)

ASUS P9D-MH Series 2-23
3. CPU Warning LED (ERR_CPU1)
The CPU warning LED lights up to indicate that a CPU error or failure has occurred.
4. Power LED (+5V_LED1)
This LED lights up when the Power-on button is pressed and the system is on.