question on subject test

Hi,
I noticed a lot of schools want a subject test with the SAT (ACT must be harder, they take that alone)My son was wondering, if you have decent SAT scores, pass your AP test and have good recs, etc., does a poor or failing grade exempt you from applying to that school? Sometimes the subjects are not from that year or not your strongest subject. He’ll probably do fine, but he was curious. Thanks if anyone knows how they are weighted.