Conscience is not the act of determining whether I judge an act to be morally good. Conscience is the act of determining whether an act is morally good.
The first sort of act, the sort of act that conscience is not, is an act I can't fail at. A genuine act of conscience, though, is something I most certainly can get wrong, both in how I do it and in what determination I make.
And if, a priori, I omit from my determination the teaching of the Church, then I most certainly am doing it wrong, even if I accidentally reach the correct conclusion.