Asus Crosshair V Formula/ThunderBolt - Manual (Page 46)

5.. GO.LED
Blinking: Indicates that MemOK! is enabled before POST.
Lighting: Indicates that the system loads the preset profile (GO_Button file)
for temporary overclocking when in OS.
6.. Q.LED
Q LEDs check key components (CPU, DRAM, VGA card, and booting
devices) in sequence during motherboard booting process. If an error is
found , the corresponding LED will continue lighting until the problem is
solved. This user-friendly design provides an intuitional way to locate the root
problem within a second.
2-16 Chapter 2: Hardware information