Check engine light -vs- catalytic converter

[97318TIgurl]

My '97 318ti failed NJ state emissions because the check engine light DID NOT come on when he started the car. My certified BMW mechanic says the bulb is there and everything looks right.

I take the car back for re-inspection, and find out that i need a catalytic converter! I take it to a second place and they connect the reader and it comes back again for needing a cat. When i questioned my mechanic, he said i don't need a catalytic converter, so now i'm ALL confused.

Could it be a wiring issue that's causing false readings? I took that car to a third place and he wants to do a 3-4 hour diagnostic to see what's going on with the car. Is this usual, and what's the normal cost for this type of diagnostic? My brother who's also a mechanic says a diagnostic should only take an hour or a little over, at best, but not 3-4.

Any assistance is greatly appreciated, as i don't want to go buying a catalytic converter if it's really not the issue. I'm ready to sell this car and just not deal with it from all the confusion.