Hello, I am asking this question because my 1993 pontiac sunbird 3.1FI car was running great until my son-n-law took it to North Carolina and the muffler and converter fell off. His mechanic and hisself put on a straight pipe instead of replacing the catalytic converter, now the car runs rough and stalls sometimes when you come to a stop. What could be the problem, if not replacing the converter is not it?