I posted this in reply to a thread in Lighting and Electrical, but I thought this info might belong here:

I've been researching this, as I have brake issues with my 93 and 94 Frankenblazers. There seems to be some common confusion, and I'll try to explain what I've learned (I think!).

Jumping terminals A and B will cause the SES light to flash and display the stored codes for the engine systems.

Jumping terminals A and H will cause the ABS light to flash the stored codes for the ABS system only.

These are two different sets of codes...a code 51 means a rear isolation valve circuit is open if it is the ABS light, and means an error in the electric control module if it is the SES light.

Check out http://www.tradervar.com/GM%20Code%20Scanner%20CP9001_english.pdf.

It is a lot to sift through, but it describes the process. BTW, most scanners only jump those terminals (but they use a nice shiny plastic case to hide what is, essentially, a paperclip) so IMHO, they are not worth the expense.