I think what zzyzxyzxxxyzyxzzzyxyxyzyyyyxyxzzxyx is talking about is that when your car gets kuff´d up, the ECU throws a code to the diagnostic connector. The diagnostic connector is a little unit in the upper, upper right hand side of the engine compartment (when looking into the bay from the front of the car). It´s by the front left strut tower, right behind it on the firewall side. It´s covered by a black lid thing and looks a little like a relay unit.
Anyway, the cap comes off and you´ll see a bunch of litle holes where the fancy diagnostic machines go and do their fancy shit.
But we live in the ghetto and we can´t afford such things.
So you can hook up a voltmeter (analog) or something like an LED or whatever and hook it up to the correct holes and those will be the feed and ground. The ECU uses these to throw the code, which is sent in morse. A series of flashes and spaces denote different codes for different problems detected by the ECU. This table of codes can be found in a haynes manual.
