I ordered a “Codemate” so I could pull any trouble codes from my ECM (1995 5.7L). There is nothing in the Codemate other than a slide switch and a led/resistor. The repair manual for MEFI states that the output from the ECM can also be as “Check engine light” on the dashboard. I butchered the Codemate and removed the led and connected an external wire and plug. I had an engraving shop make me two faceplates for the led, one white and one gray. I drilled a hole in my dash and tried both faceplates and decided they just looked too obvious so I installed the led by it self. If I ever do have a fault in the system, all I have to do is slide the switch to the service position and check the code. The led flashes “12” 3 times and then the fault code.