According to the DECstation 2100/3100 Operator's Guide, these are the
self-test codes: 

7.	Video SIMM
6.	VDAC and PCC
5.	DZ serial line
4.	System Module (it notes that you should follow SCSI
	  troubleshooting procedures)
3.	Keyboard and mouse
2.	Network interface
1.	Disk and SCSI bus peripherals
0.	All tests succeeded

Also, in case they're helpful, here are the codes for the LED status
display on the back of the machine:

0111 1111	(0x7f)		System module
1011 1111	(0xbf)		Disk
1101 1111	(0xdf)		Keyboard
1110 1111	(0xef)		Memory
1111 0111	(0xf7)		Mouse
1111 1111	(0xff)		"LED's set at power up or reset"
				(i have no idea what that means) :) 

				eric