Don’t worry if the Check Engine Light is not on, the computer will store lesser important codes without flashing lights in your face. First you need to find the self-test plugs to read codes from the EEC. There are 2 plugs to find. One is triangular and has 6 pins; it is called the Self-Test plug. The other is a smaller single pin plug; it is called the Self-Test Input (STI). These are what the plugs look like. The self-test plugs are located along the driver's side fender by the firewall on 1986-95 Mustangs.
 
 
 
 
The self-test plugs are located along the driver's side fender behind the air filter box on 1987-95 EFI trucks.
 

The self-test plugs were mounted on the passenger side fender on 1985-86 EFI trucks.

How to Prepare for the Self-Test

To check for codes without buying a code reader is simple. If your EEC is wired correctly, the engine light works, and there are not any electrical faults in the vehicle you can get the codes out of your EEC.

1. Turn key to OFF.
2. Get a length of wire that can connect the STI to the negative battery terminal or body ground.
3. Ground the Self-Test Input.
4. Get some paper and a pen.
5. Get comfortable in the drivers seat.
6. Try to keep your self from getting distracted from here on.
7. Turn key to RUN.
8. You will hear some relays engage and the fuel pumps prime the system. This is the EEC beginning the test.
9. When the fuel pumps stop be ready to count!

If you are a lost soul that does not have a working engine light, or did not install a light into your dash don't fret. You can still get the codes. You still need that wire to ground out the STI. Now you will need one of the following tools; digital volt meter, analog volt meter, test light, or you can hook up any other 12 volt light that does not draw more than 0.5 watts max.

1. Turn key to OFF.
2. Get a length of wire that can connect the STI to the negative battery terminal or body ground.
3. Ground the Self-Test Input.
4. Attach one side of the test device to the Self-Test Output (STO).
5. Attach the other lead to the positive side of the battery.
a) If you are using a digital voltmeter you'll want to use the continuity setting.
b) If you are using an analog voltmeter you'll be counting the meter make sweeps.
c) If you are using a cheep light you'll count like the rest of us.
6. Get some paper and a pen.
7. Get comfortable in the drivers seat.
8. Try to keep your self from getting distracted from here on.
9. Turn key to RUN.
10. You will hear some relays engage and the fuel pumps prime the system. This is the EEC beginning the test.
11. When the fuel pumps stop be ready to count!

How To Count “Check Engine” Light Blinks

When the “Check Engine” light blinks during the test you will get several different blinks, the separation time between blinks is used to indicate what information is being displayed.

Turn key to RUN.
You will hear some relays engage and the fuel pumps prime the system. This is the EEC beginning the test.
The fuel pumps will stop and the EEC will now begin giving codes.
You may catch a brief flickering of the light. This is a rapid code read that is used by the code readers you can purchase. Do not worry all those rapid flashes you just say flicker by will now be given at a slower pace of 1/2, 2, & 4 second pauses.
First digit of first KOEO code (1/2 second between blinks)
2 second pause
Second digit of first KOEO code (1/2 second between blinks)
4 second pause
Next KOEO code
Once all KOEO codes are read the EEC will repeat them
6 second pause
Single Separator Blink (this indicates the change from KOEO codes to CM codes)
6 second pause
Codes from Continues Memory (uses same 1/2, 2, and 4 second timing as KOEO codes and also repeats them)
If nothing has been recorded by the EEC since last battery disconnect you will see:

Blink – 2 sec – blink – 4 sec – blink – 2 sec – blink – 6 sec – blink – 6 sec – blink – 2 sec – blink – 4 sec – blink – 2 sec - blink

This translates into 11-11 current condition (KOEO) and 11-11 in the continues memory (CM); or SYSTEMPASS

This may seem hard to count; recommendation is to record codes with slashes, and to lump them all together. Sort them out into numerical codes later. Here is an example of this:

This would give you codes; 41, 83, 14 & 18. If you miss some of the codes or feel you are counting wrong continue to count the remaining codes for practice and to compare to the next code test. When finished simply turn the key to OFF wait 15 seconds then back to RUN. The codes will be presented again.

Complete list of 2-Digit Self-Test Codes and their definitions.
Complete list of 3-Digit Self-Test Codes and their definitions.
If you can not get codes; read about Solving a No Code Problem here.