Ok, so recently we've had a lot of questions about how to do this. MrShorty has a great How To on this already but it seems it's a little advanced for some people, so I'm going to make a simple "How To" with pics. This isn't a full how too, this is just to get you started, for the advanced stuff I'm going to link to MrShorty's post!!
----There is now also a video at the bottom of this post made by our own Joe Dirt, it details the process as well. So if you like video over pictures, or are still unsure, check it out!!!!----
*** This should work on 1st Gen 91-94 Explorers and also 95 2nd Gen. The 95 got a face lift but uses the same 1st Gen brain. From what I read if you have a 96+ you HAVE to have a code reader.****
So you've got a CEL light that likes to wink at ya, and you don't have a code reader!!! NO PROBLEM! All you need is a paper clip (or a bit of wire), your keys, your eyes, and your Check Engine Light.
First you have to locate your EEC test harness... It's back behind the Power Dist box on my 93...I've heard it's on the other side on some 1st gens, so I'm hoping someone will post up some pics for those models. It's the black box to the left of the air box in this picture...
What your looking for is a harness that plugged into the EEC TEST holder attached to the back of that black box. You pull the harness DOWN out of the holder, the holder stays attached to the Power Dist box. If the harness isn't in the holder (I don't put mine back) then it's laying loose just right behind this box.
Now this is what the harness your looking for looks like, you need both the black large one, and the single gray one....
You'll need a paper clip, or do like I do and use a piece of wire....
Now your going to "jump" the #2 slot in the main harness with the single gray harness like so...
Now you just get in the truck, turn the key, and watch this thing flash the codes at ya....
That's it, your done, and you now can get your Check Engine Light Codes from your 1st gen Explorer.
***Word of advise/warning... pulling the jumper off while the key is on (test mode) will erase all memory of all codes from the system. It's the same as leaving the battery unhooked to clear the codes. Don't pull the jumper during the test unless erasing the codes is what you are trying to do. It won't damage anything really, and is how I clear MY codes. Just be aware you'll lose your codes if you pull it during the test.****
I'm going to go over this REAL fast, then link you to MrShorty's post for the real deal. This is the process for the KOEO test (Key On, Engine Off). Get a piece of paper and a pen, make sure your jumper is in place, turn the key to "on" but DON'T start the truck. You'll start hearing some relays clicking and the fuel pump whine and after a few moments your CEL will "flicker" that's the "fast codes" out putting, those are for code readers and you ignore that "flicker". Then the real codes will start. The first codes will be any faults that are happening while the Key is on, but the engine is off. For example if you had a 157 fault from the MAF for example the CEL would flash once....*small pause* Five times *small pause* Seven times *small pause*, then repeat a second time (157). If you had two codes like 157 AND 158 it would flash both codes the repeat them both (157,158,157,158).
After the last code it will pause about *6 seconds* then flash once to tell you it's getting ready to give you the CM Codes. (that one flash is called the seperator flash) These are the codes that get thrown while driving but go away. In other words, if your driving and the CEL comes on, then goes off, this it where you retrieve that code. It will process just like above.
You will only get codes in the first part of the output phase (before the CM codes) if there is a CURRENT KEY ON FAULT, other wise you will get 111, that means everything is "ok" with just the key on, no faults the CPU could find. It will repeat just like normal codes. So lets say there are no current faults, but that the MAF went "high" while driving (158), this is what will be output... *flicker fast codes* (start of KOEO codes) 111,111 *6 second pause* 1 (seperator flash) *another pause* (start of CM codes) 158, then a repeat of 158, then no more flashes.
It takes some practice to get used to. But it's easy once you've done it. Just use a piece of paper and copy each code as it flashes, then figure out what you've got once you finish.
Ok, so here is the advanced instruction, and more general info on how to read the codes, and also how to do the KOER (Key On, Engine Running) test.
MrShorty's awsome thread about pulling codes!
And here is a link to what the codes MEAN!
Brooklynbay's list of OBD I CEL codes!
Here is a great video about how to do this made by our own Joe Dirt, enjoy!!!!