The last time I got it scanned by a computer, catalytic converter came up, but the mechanic told me it's nothing important. Infact he told me to rip it off! I did have a vacuum leak but that was because of a hose. Found out why my engine kept overheating, my oil filter was extremely loose, about to fall off! Had an oil change and my mechanic tightened that sucker, so far no over heating. Apparently it was the fact that some of the pistons weren't getting enough oil.
So, the scanner said the catalytic converter has problems...did you remove it like your mechanic suggested?
If that filter was so loose that it was ready to fall off, it's likely that there wasn't any oil left in the engine. Maybe it's not worth saving after all?