Why Parts?

First they changed the catalytic converter to solve the problem of sulphur smell in the exhaust, after few days problem was back, so they changed the
air flow sensor, problem persists so engine timing, problem persists etc..etc...

In short the car went back 7 times for the same problem of bad exhaust smell, ( I wonder what is the lemon law for used car?)

Problem went away for about six months, but is back now occassionally.