I just bought a 91 f250 with a 460 / E4od. Its in decent shape and all. I want to check the codes and I cant for the life of me, find the self test connector Ive checked on the driver side fender well, and firewall. All along the fire wall, and the driver side fender well. Ive also looked under the dash. I cannot find it. Any ideas guys? Thanks. Im really happy with the truck. Not long ago I had an 88 f250 woth 7.3 dsl. I foolishly sold it to a guy to help him out. Then got a 3/4 ton chevy later. It didnt take long for me to miss the heavy, solid engineering of my old f250 hd. That chevy felt like a twisty, weak, flexible excuse for a truck. It also rattled like a spray paint can. Junk......
Im back in business now. Had to have another f250 hd. They just dont make em like that anymore. Thick, heavy, solid, and easy to work on.
I dont plan to sell this one. Knowing that I would be missing it 2 weeks later.

 

Ford Fuel Injection » How To Run a Self-Test

 

also most likely (seeing as you said this is a 91) your also going to have OBD1 and there should be a port inside the engine compartment driver side bay area, should have a funny shape to it, that will be your OBD1 "test" port for any scanner you have to hook into (ill post a pic next time in under the hood of my truck to show you if you need it)