When power is first applied, initial self tests are performed by built it ROM routines and special processor on the system planar board.  These self tests should complete with in 1-2 minutes. The tests labeled 'fatal' indicate that if this code remains for more than five minutes, indicates that component has failed.
   Numbers display on the two-digit display to track the progress of the POST programs. Some tests may run so quickly that the numbers may not be visible.

 Note: I have seen tests 14,15,16 and 17  last for up to three to four minutes, depending on the number of hard disks and/or floppy drives  connected to the installed adapters. (MW)

NOTES:
 The following chart is for "steady" numbers only. Flashing (about one second) numbers are for software failures. See page 2-19 for flashing numbers during the diagnostic checkouts.
   When u is shown in the chart, it is displayed in the upper half of the two-digit display.
    IBM RT PC 6151 System Unit Hardware Maintenance and Service, pgs 2-6 thru 2-8.
 

ROS  LED	Values during IPL
88	Displays for 1 sec (lamp test). If steady, the 32-bit processor has stopped. reserved - POR or processor check
00 0c	Initialization of Mono/Printer adapter failed  Initialization failed - Mono/Printer adapter and APC
01 1c 1u	ROS CRC did not compare -  - Fatal Advanced Processor card-  - Fatal Enhanced Advanced Processor and  - Fatal memory Mgmt card
02	Tests for correct communications between 32-bit processor and the system board. Soft IPL check failed - Fatal
03 3c 3u	Memory error or no memory -  - Fatal Advanced Processorcard and first 128K of memory. - - Fatal Enhanced Advanced Processor - Fatal Memory Mgmt card.
04 4c 4u	Processor card logic error -  - Fatal APC logic (AdvancedProcessor Card).  - Fatal Enhanced Advanced Processor  - Fatal Memory Mgmt card Logic.
05 5c 5u	Processor or memory error condition -  - Fatal APC logic and system memory.  - Fatal Memory expansion option / Enh Adv Processor  - Fatal Memory managment card.
07	IOCC test resident POST - Fatal
08 8c 8u	Bad processor card in IOCC test -  - Fatal IOCC testfor APC.  - Fatal Enhanced Advanced processor - Fatal Memory managment card.
09	Keyboard adapter resident POST - Fatal
10	System timer resident POST - Fatal
11	Interrupt controller POST - Fatal
12	DMA arbiter resident POST - Fatal
13	Serial port resident POST
14	Fixed disk resident POST adapter 1 test
15	Fixed disk resident POST adapter 2 test
16	Diskette resident POST adapter 1 test
17	Diskette resident POST adapter 2 test
Beep	Tests Speaker on keyboard
18	Extension ROS
19	Attemting Manufacturing IPL
20	NVRAM CRC check
21	Attempting to IPL from devices stored in battery- powered memory. No boot record found (NVRAM selected devices)
22	Attempting to IPL from default devices stored in ROM on processor board. No boot record found (ROM selected devices)
23	No IPL devices in battery-powered memory or the ROM default list could be identified.  Disk or diskette adapter slot or address
25	Error occured during IPL. System must be powered off to clear this error. User error - invalid RamSpecReg - Fatal
26	Attempting soft IPL
27	Bootable code exceeds available storage - Fatal
28	Unexpected return from loaded code - Fatal
29	IPL process passed control to loaded code.
30-uu	Individual adapter or device tests
89	Unexpected machine or program check - Fatal.
96	Memory card 1 resident POST error condition.
97	Memory card 2 resident POST error condition.
98	Memory card 1 and 2 resident POST error condition.
99	Testing stopped Keylock in Locked position.