How to Read Ford EEC-IV Self-Test Codes
What this covers
Ford called its system EEC-IV. It runs its own self-test and blinks the codes on the check-engine light, the same idea as GM but a different connector and a different test. The car does the test itself once you start it. You watch and count.
Find the self-test connector
Ford put a separate self-test connector under the hood, not under the dash. It is a set of small connectors near the firewall or a fender, often beside the main harness. You are looking for the single-pin self-test input, called STI, and the signal return pin, called SIG RTN. On some trucks they sit apart, on others in one plastic housing.
Jumper STI to signal return
Key off. Bridge the self-test input to signal return with a jumper wire. That tells the computer a test is being requested. Turn the key to run with the engine off. The computer runs the key-on engine-off test, called KOEO, and blinks the results on the check-engine light.
Watch the KOEO test
The KOEO test checks the sensors and circuits that can be seen with the engine stopped. The light flashes the stored trouble codes first, then a separator pulse, then the hard codes found in this test. Count every flash group and write it down exactly as it blinks.
Run the engine test next
When the KOEO test finishes, start the engine and let it idle. The computer runs the key-on engine-running test, called KOER. It flashes a cylinder identification pulse first, so you know to snap the throttle when asked, then it blinks the running codes. Stay clear of the fan and belts.
Read the codes
Ford codes are usually two digits. The light flashes the first digit, pauses, then flashes the second. Write the number and the test down together, because the same number can mean different things in KOEO and KOER.
Clear the code and recheck
Fix the fault first, then disconnect the jumper and clear the stored codes by pulling the computer fuse or disconnecting the battery for a few minutes. Run the self-test again after a repair. If the code returns, the fault is still there. The Ford OBD-I code list gives the meaning, the causes, and the check order for each code.