Asus CROSSHAIR IV EXTREME User Manual page 31

7. POST State LEDs
POST State LEDs check key components (CPU, DRAM, VGA card, and
HDD) in sequence during motherboard booting process. If an error is found,
the LED next to the error device will continue lighting until the problem is
solved. This user-friendly design provides an intuitive way to locate the root
problem within seconds.
ROG Crosshair IV Formula 2-5