Mazda Allegro 1.6

I have a problem, it turns out that a few days ago the witness of the bacteria was lit, well as it should be, where the mechanic resisted him from arriving at the point that the alternator was fought with the asta that they could no longer reach the point of changing it, the bacterium the same is new only has 2 months they reviewed wiring and the witness is still on, they told me that it was the ECU and that I have to change it. Make thanks