'99 Chevy Lumina - ECM?

We are not entirely sure of the problem. The car will constantly display a service engine soon light because of a bad knock sensor. The sensor, connector and wiring have all been replaced but the light didn’t go off. This has been the case since last march when it needed inspected. Our mechanic, and my dad’s friend that owns a garage, managed to inspect the car because the light miraculously went out for a few minutes. He told us that it was the computer that was bad and we have just went by his word since then.

The other things that the car will do is overheat because the secondary fan will not turn on but when you connect current to it, it will run.