1 4 is motherboard failure

The diags don't run if the cpu has failed.

leds

PC post codes go back to 1971 and the INTEL 4004.

MSDOS came out in 1981  before that the "DOS" was CP/M

POST ERROR Codes

A POST code card can read these as well because there is a standard to write the codes to port 80.

This assumes you know how to design motherboards and bios and use INTEL chipsets and processors.

https://www.intel.com/content/www/us/en/support/articles/000005500/boards-and-kits.html

a very long time ago dell used modified INTEL Desktop boards for their PentiumII and Pentium 3  Dimension 4100 for example.

In computing, a POST card is a plug-in diagnostic interface card that displays progress and error codes generated during power-on self-test (POST) of a computer. It is used to troubleshoot computers that do not start up.

This Interactive POST Code Diagnostics Decoder is intended to assist in troubleshooting a system hang during a system's Power-On Self-Test (POST) process. Intel Server Boards include a bank of eight POST Code Diagnostics light-emitting diodes (LEDs) at the back edge. As each routine is started, the given POST code number is displayed to these LEDs.

https://www.intel.com/content/dam/support/us/en/multimedia/server-products/server-boards/7059/

 

https://www.amazon.com/Hariier-Multifunction-Motherboard-Diagnostic-diagnostic/dp/B07FY5285V

Port 80h POST code ranges

In the tables below, all POST codes and range values are listed in hexadecimal.

Range	Category/Subsystem
00 – 0F	Debug codes: Can be used by any PEIM/driver for debug
10 – 1F	Host Processors
20 – 2F	Memory/Chipset
30 – 3F	Recovery
40 – 4F	Reserved for future use
50 – 5F	I/O Busses: PCI, USB, ISA, ATA, and so forth
60 – 6F	Not currently used
70 – 7F	Output Devices: All output consoles
80 – 8F	Reserved for future use (new output console codes)
90 – 9F	Input devices: Keyboard/Mouse
A0 – AF	Reserved for future use (new input console codes)
B0 – BF	Boot Devices: Includes fixed media and removable media.
C0 – CF	Reserved for future use
D0 – DF	Boot device selection
E0 – FF	E0 - EE: Miscellaneous codes F0 – FF: FF processor exception

Port 80h POST codes

POST Code
00 - 0F	Debug Codes: can be used by any PEIM/driver for debug
10 - 1F	Host Processor
10	Power-on initialization of the host processor (bootstrap Processor)
11	Host processor Cache initialization (including APs)
12	Starting Application processor initialization
13	SMM initialization
14	LAN initialization
15	Exit early platform init driver
16	SMBUS driver initialization
17	Entry to SMBUS execute read/write
19	Entry to CK505 programming
1F	Unrecoverable CPU error
20 - 2F	Memory / Chipset
21	Initializing a chipset component
22	Reading SPD from memory DIMMs
23	Detecting presence of memory DIMMs
24	Programming timing parameters in the memory controller and the DIMMs
25	Configuring memory
26	Optimizing memory settings
27	Initializing memory, such as ECC init
28	Testing memory
2F	No memory detected or no useful memory detected
30 - 3F	Recovery
30	Crisis Recovery has initiated per User request
31	Crisis Recovery has initiated by software (corrupt flash)
34	Loading recovery capsule
35	Handing off control to the recovery capsule
3F	Unable to recover
50 - 5F	I/O Busses (PCI, USB, ISA, ATA, and so forth)
50	Enumerating PCI busses
51	Allocating resources to PCI bus
52	Hot Plug PCI controller initialization
53–57	Reserved for PCI Bus
58	Resetting USB bus
59	Reserved for USB
5A	Resetting PATA/SATA bus and all devices
5B	Reserved for ATA
5C	Resetting SMBUS
5D	Reserved for SMBUS
5F	Unrecoverable I/O bus error
60 - 6F	There are currently no valid POST codes in the 60 - 6F range. If you see a POST code in this range, read it as B0 - BF. (On a 7-segment LED display, the b looks like a 6.) Example:
70 - 7F	Output Devices
70	Resetting the VGA controller
71	Disabling the VGA controller
72	Enabling the VGA controller
78	Resetting the console controller
79	Disabling the console controller
7A	Enabling the console controller
7F	Unrecoverable output device error
90 - 9F	Input Devices
90	Resetting keyboard
91	Disabling keyboard
92	Detecting presence of keyboard
93	Enabling the keyboard
94	Clearing keyboard input buffer
95	Instructing keyboard controller to run Self Test (PS2 only)
98	Resetting mouse
99	Disabling mouse
9A	Detecting presence of mouse
9B	Enabling mouse
9F	Unrecoverable input device (keyboard or mouse) error
B0 - BF	Boot Devices
B0	Resetting fixed media
B1	Disabling fixed media
B2	Detecting presence of a fixed media (IDE hard drive detection and so forth)
B3	Enabling/configuring a fixed media
B8	Resetting removable media
B9	Disabling removable media
BA	Detecting presence of a removable media (IDE, CD-ROM detection, and so forth)
BC	Enabling/configuring a removable media
BF	Unrecoverable boot device error
D0 - DF	Boot Device Selection
Dy	Trying boot selection y (y= 0-15)
E0 - FF	Miscellaneous Codes
E0	Started dispatching PEIMs (emitted on first report of EFI_SW_PC_INIT_BEGIN EFI_SW_PEI_PC_HANDOFF_TO_NEXT)
E2	Permanent memory found
E1, E3	Reserved for PEI/PEIMs
E4	Entered DXE phase
E5	Started dispatching drivers
E6	Started connecting drivers
E7	Waiting for user input
E8	Checking password
E9	Entering BIOS setup
EB	Calling Legacy Option ROMs
EF	Boot/S3 resume failure
F4	Entering Sleep state
F5	Exiting Sleep state
F8	EFI boot service ExitBootServices ( ) has been called
F9	EFI runtime service SetVirtualAddressMap ( ) has been called
FA	EFI runtime service ResetSystem ( ) has been called
FF	Processor exception

Typical port 80h POST sequence

Port 80h code values typically increase during the boot process. The early codes are for subsystems closer to the processor and the later codes are for peripherals. Generally, the order of initialization is Processor -> Memory -> Busses -> Output/Input Devices -> Boot Devices. The sequence of POST is system-specific.

POST code	Description
21	Initializing a chipset component
22	Reading SPD from memory DIMMs
23	Detecting presence of memory DIMMs
25	Configuring memory
28	Testing memory
34	Loading recovery capsule
E4	Entered DXE phase
12	Starting Application processor initialization
13	SMM initialization
50	Enumerating PCI busses
51	Allocating resourced to PCI bus
92	Detecting the presence of the keyboard
90	Resetting keyboard
94	Clearing keyboard input buffer
95	Keyboard Self Test
EB	Calling Video BIOS
58	Resetting USB bus
5A	Resetting PATA/SATA bus and all devices
92	Detecting the presence of the keyboard
90	Resetting keyboard
94	Clearing keyboard input buffer
5A	Resetting PATA/SATA bus and all devices
28	Testing memory
90	Resetting keyboard
94	Clearing keyboard input buffer
E7	Waiting for user input
01	INT 19
00	Ready to boot