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 take it where the mechanic checked him from arriving at the point that the alternator fought with the asta that they could not reach the point of changing it, the bacterium the same is new only has 2 months they checked wiring and the witness is still on, they told me that it was the ECU and that I have to change it. thank you