The following is a list of reasons why you may see the MIL (CEL) lit with no accompanying Continuous Memory Self-Test codes:

(1) Unfamiliarity With Self-Test Code Output.
There are two types of EEC Self-Tests, Key On Engine Off (KOEO) and Key On Engine Running (KOER). While both of these will test for various "hard faults" that are present when the test is run, the processor continuously monitors various operating parameters whenever the engine is running. If the processor detects a problem, it will store a "Continuous Memory" code and light the MIL. These Continuous Memory codes are put out during KOEO Self-Test after any codes associated with hard faults are output.

Self-Test Codes are displayed by flashing the MIL. They are also output as voltage pulses on the Self-Test Output (STO) circuit in the Self-Test connector. In either Self-Test mode, all codes are output twice and in KOEO, the hard fault codes are separated from the Continuous Memory codes by a "separator" pulse. A technician that is unfamiliar with the EEC Self-Test can mistakenly believe that continuous Memory codes are not present when they really are. He may run KOER Self-Test and get a pass code (11/111) and not realize that KOEO Self-Test must be run to receive any Continuous Memory codes. He may run KOEO Self-Test while counting MIL flashes and misinterpret the repeated hard fault pass code (11/111) to mean that Continuous Memory does not contain any codes.

(2) Inadvertent Erasure Of Continuous Memory Self-Test Codes.
Continuous Memory Self-Test codes are erased by ungrounding STI before the KOEO Self-Test is complete and all KOEO and Continuous Memory codes have been displayed. It is possible to inadvertently erase Continuous Memory codes by ungrounding STI without realizing that KOEO Self-Test is incomplete or the processor has not finished displaying all the codes. If STI is ungrounded before all slow codes are output, Continuous Memory will be erased and a pass code (11/111) could be displayed the next time KOEO Self-Test is run.

(3) The Concern That Set The Continuous Memory Code Is No Longer Present.
The EEC processor will erase a Continuous Memory code if the concern that caused it has not been present for 40 or 80 warm-up cycles, depending on the vehicle. A warm-up cycle occurs when the vehicle is started with the coolant temperature below 120° F (49° C) and then shutdown with the coolant temperature above 150° F (66° C). If a vehicle is brought in for service with a MIL complaint and the vehicle is driven or otherwise allowed to warm-up before Self-Test is run, the code may be cleared before the technician tests it.

(4) Grounded STO/MIL Circuit.
The processor controls the MIL by grounding the STO/MIL circuit (Pin 17). If this circuit shorts to ground, whether the processor is controlling it or not, the MIL will be lit. Starting in 1991, if the processor has lit the MIL, it will hold it on for a minimum of 10 seconds. If the MIL flashes quickly, the concern is probably the STO/MIL circuit shorting intermittently to ground.

(5) Engine Running In HLOS.
The EEC processor will enter Hardware Limited Operation Strategy (HLOS) if it detects a problem that could cause further damage to the system. Under HLOS, the processor modifies its operating strategy so that certain functions are disabled but the vehicle can be safely driven in for service. If the vehicle is in HLOS, Continuous Memory codes will not be set and Self-Test cannot be initiated. However, Continuous codes that were set before the processor entered HLOS will be retained.

6) Misinterpretation Of MIL Bulb Check.
The MIL will light as a bulb check if the key is on and the engine is not running. If the engine is running and stalls or stops for any reason with the key on, the MIL will be lit and no Continuous Memory codes will be set. When the key is first turned on, the MIL will stay lit briefly after the engine is started as part of the bulb check feature.

(7) MIL Flashes During Self-Test
The circuit that controls the MIL is also the Self-Test Output (STO) circuit that goes to the Self-Test connector. The MIL will flash during Self-Test as the STO circuit is cycled on and off. This is normal and no Continuous codes are set.

(8) Processor KAM Is Erased Or Fails.
The Keep Alive Memory (KAM) within the processor must always have voltage supplied to it. This voltage is supplied by the Keep Alive Power (KAPWR) circuit (Pin 1) that connects directly to the battery. KAM contains adaptive parameter tables that allow the processor to adapt to different operating requirements. It also contains the Continuous Memory codes. Continuous Memory codes will be erased any time KAPWR is disconnected (i.e. battery disconnected, processor disconnected, breakout box installed, open in the wire, etc.). If KAM fails within the processor, all Continuous codes will also be erased.

(9) Damaged STAR Tester.
A damaged STAR tester can produce erroneous code output or accidentally erase Continuous Memory.

(10) KOEO Processor RAM Test Failed.
The processor's Random Access Memory (RAM) is tested during KOEO Self-Test. If the processor's RAM has failed, the MIL will light and no codes are output.

(11) Intermittent VSS Fault Detected In Wiggle Mode.
If in wiggle mode (STI grounded) and an intermittent Vehicle Speed Sensor (VSS) fault is detected, the MIL can be lit momentarily. If the VSS signal returns to normal, the associated code is erased. In normal operation, the VSS will not light the MIL.

(12) IDM Pulsewidth Not Recognized By Processor (EDIS Vehicles).
EDIS vehicles can have the MIL on with no Continuous codes if the processor does not recognize the Ignition Diagnostic Monitor (IDM) pulsewidth. In this case, coil pack failure codes may not be set since the fault filters can be erased before they reach the threshold that sets the code.

(13) Intermittent Ignition System Fault.
Vehicles with a Cylinder Identification (CID) sensor can light the MIL with no Continuous codes present if an intermittent ignition system fault is present long enough to activate the MIL and then goes away. The CID sensor can indicate that the fault was momentary and clear the coil pack faults but the CID fault may not register if the fault goes away fast enough.

(14) Intermittent Open STI Circuit.
If the Self-Test Input (STI) circuit opened during KOEO Self-Test or code output, Continuous Memory would be cleared.

(15) Power Lost To EEC Processor.
On some applications, the processor can lose power while the MIL stays powered. The MIL can light if a ground path is present through the processor.

(16) Other Warning Lamps Mistaken For MIL
The MIL can sometimes be confused with other warning lamps like the amber Air Bag lamp if they are located near each other in the dash panel.

(17) Development Testing Or Wrong Processor Released To Production
The MIL can be lit without Continuous codes during testing or if the wrong processor is installed

Source: Ford TSB/Article No. 92-24-3