I just had to buy a new one for my 94 Cobra. It ran very poorly, had bad throttle lag, no mid range pull, idled rough. As I was explained, it takes all the inputs from various engine control system such as MAF, and TPS, and runs the engine correctly.
I got code 511 using the EEC tester in the engine compartment, and got the computer used on ebay, worked like a charm
mine started with the over-heat problem all of a sudden. then my fuel pump stops and my fan goes on high speed when my key is on the ON position (not started yet). i pluged my tweecer and red the EEC i found a very strange numbers ( way off the stock). i changed my computer today and every thing went back to normal
Computer seldom fail, most often it is a sensor or actuator that quits working.
Te first step in checking the computer is to dump the codes.
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.
IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.