Mazda 2004 Code P0421

I just purchased a Mazda 04 and the engine went off recently with a code of P0421 with the following possible causes:

-Exhaust leak
-Check other service codes first
-Faulty catalytic converter
-Faulty rear HO2S (Heated Oxygensensor)

I'm trying to figure what what the issue is, the carruns fine but has the following diagnostics:
-Steering wheel vibrates at 60mph+ (probably wheel alignment issue)
-Smell of gasoline inside car

I'm thinking it maybe an exhaust leak because of the smell inside the car but am not sure if it maybe the CAT that needs to be replaced, also not sure what it may cost to replace it.

Car Specs:
Make: Mazda
Year: 2004
Mileage: 100k
Code: P0421

If you have a smell that seems to be exhaust or unburned fuel, by all means figure out the source before changing out O2 sensors or converters.

As for the vibration in the steering wheel, wheel alignment will not cause the sensation of a vibration. The source of the vibration could be a bad tire, bad wheel bearing, bad half shaft, or worn steering or suspension parts. If you find a bad tire is causing the vibration, review the tire wear. An alignment could be the root cause as to why the tire is wearing, as could worn steering or suspension parts.

-Rod

P0421 is usually a bad converter. As for a fuel smell, that definitely needs to be figured out. It should be easy enough, maybe someone filled a gas can and spilled it in the car. As for the vibration, good money goes on it needing a tire balance. It could be a bad tire but more than likely it has lost a wheel weight. An alignment won't cause a vibration.