Update (10/5/2021): Added Z590 debug codes. They're completely same as Z390 debug codes.
Gigabyte Z390 and Z590 motherboards come with an helpful debug LED that helps troubleshooting system issues. The debug LED is located bottom of the motherboard and helps you to pinpoint the source of the problem, in case your system doesn't boot or the monitor is blank. Such problems might occur because of many reasons like incompatible / faulty hardware, extreme overclocking settings, unplugged cables, wrong BIOS settings or not fully insterted RAM, etc.
During the boot, the LED rapidly shows various hexadecimal values, with every value representing a successful event or an error. Therefore the LED showing some value doesn't necessarily mean there is a problem. If your system can't boot, check the current LED value and compare it with the list below.
For more troubleshooting, refer to Gigabyte troubleshooting guide.
Note: We have taken the following list from the Gigabyte Z390 Aorus Ultra and Z590 Aorus Master manual. These values probably mean the same for the other Aorus models in same line, however I cannot confirm it since I don't have their manual.
Z390 Aorus Ultra & Z590 Aorus Master debug LED codes
10 PEI Core is started.
11 Pre-memory CPU initialization is started.
15 Pre-memory North-Bridge initialization is started.
19 Pre-memory South-Bridge initialization is started.
2B~2F Memory initialization.
31 Memory installed.
32~36 CPU PEI initialization.
37~3A IOH PEI initialization.
3B~3E PCH PEI initialization.
60 DXE Core is started.
61 NVRAM initialization.
62 Installation of the PCH runtime services.
63~67 CPU DXE initialization is started.
68 PCI host bridge initialization is started.
69 IOH DXE initialization.
6A IOH SMM initialization.
70 PCH DXE initialization.
71 PCH SMM initialization.
72 PCH devices initialization.
73~77 PCH DXE initialization (PCH module specific).
78 ACPI Core initialization.
79 CSM initialization is started.
7A~7F Reserved for AMI use.
80~8F Reserved for OEM use (OEM DXE initialization codes).
90 Phase transfer to BDS (Boot Device Selection) from DXE.
91 Issue event to connect drivers.
92 PCI Bus initialization is started.
93 PCI Bus hot plug initialization.
94 PCI Bus enumeration for detecting how many resources are requested.
95 Check PCI device requested resources.
96 Assign PCI device resources.
97 Console Output devices connect (ex. Monitor is lighted).
98 Console input devices connect (ex. PS2/USB keyboard/mouse are activated).
99 Super IO initialization.
9A USB initialization is started.
9B Issue reset during USB initialization process.
9C Detect and install all currently connected USB devices.
9D Activated all currently connected USB devices.
A0 IDE initialization is started.
A1 Issue reset during IDE initialization process.
A2 Detect and install all currently connected IDE devices.
A3 Activated all currently connected IDE devices.
A4 SCSI initialization is started.
A5 Issue reset during SCSI initialization process.
A6 Detect and install all currently connected SCSI devices.
A7 Activated all currently connected SCSI devices.
A8 Verify password if needed.
A9 BIOS Setup is started.
AB Wait user command in BIOS Setup.
AD Issue Ready to boot event for OS boot.
AE Boot to Legacy OS.
AF Exit Boot Services.
B0 Runtime AP installation begins.
B1 Runtime AP installation ends.
B2 Legacy Option ROM initialization.
B3 System reset if needed.
B4 USB device hot plug-in.
B5 PCI device hot plug.
B6 Clean-up of NVRAM.
B7 Reconfigure NVRAM settings.
E0 S3 Resume is started (called from DXE IPL).
E1 Fill boot script data for S3 resume.
E2 Initializes VGA for S3 resume.
E3 OS S3 wake vector call.
F0 Recovery mode will be triggered due to invalid firmware volume detection.
F1 Recovery mode will be triggered by user decision.
F2 Recovery is started.
F3 Recovery firmware image is found.
F4 Recovery firmware image is loaded.
F5~F7 Reserved for future AMI progress codes.
50~55 Memory initialization error occurs.
56 Invalid CPU type or speed.
57 CPU mismatch.
58 CPU self test failed or possible CPU cache error.
59 CPU micro-code is not found or micro-code update is failed.
5A Internal CPU error.
5B Reset PPI is failed.
D0 CPU initialization error.
D1 IOH initialization error.
D2 PCH initialization error.
D3 Some of the Architectural Protocols are not available.
D4 PCI resource allocation error. Out of Resources.
D5 No Space for Legacy Option ROM initialization.
D6 No Console Output Devices are found.
D7 No Console Input Devices are found.
D8 It is an invalid password.
D9~DA Can't load Boot Option.
DB Flash update is failed.
DC Reset protocol is failed.
E8 S3 resume is failed.
E9 S3 Resume PPI is not found.
EA S3 Resume Boot Script is invalid.
EB S3 OS Wake call is failed.
F8 Recovery PPI is invalid.
F9 Recovery capsule is not found.
FA Invalid recovery capsule.
Source: Z390 Manual (pictures below), Z590 Aorus Master Manual