I have a 2001 Nissan Pathfinder 2wd
Is this the sensor or catalytic converter? If so, which one?

The code reads "Catalyst system efficiency below threshold-bank 2"

Is bank 2 the driver's side?

I don't think its the EMC issue other wise I would be getting both codes...

Thanks

Update:replaced gaskets, 02 sensor same code. Before I replaced a thing, I put in 93 grade fuel combined with Lucas fuel injector and didn't get the code for 150 miles. Just thought Id throw that out there since code comes on within 10 miles. Gas mileage is still bad, get the occasional hint of rotten egg smell.

Can you tell if a cat is bad just by looking at the inside of it (that's what mechanic did)

Bank 2 is the US driver's side.

FSM says possible causes are:

Warm-up three way catalyst
I Exhaust tube
I Intake air leaks
I Injectors
I Injector leaks
I Spark plug
I Improper ignition timing

Chuck nailed it, fast way to know is get the exhaust gasses metered and diagnosed, otherwise the things that could be wrong will take a very long time to fix without knowing what to fix.

My mechanic checked the catalytic converter, said it looked good and wasn't damaged. Next was the O2 sensor next to the cc. Replaced that, still get the code. Now he is saying its the muffler gaskets...

Update:replaced gaskets, same code. Before I replaced a thing, I put in 93 grade fuel combined with Lucan fuel injector and didn't get the code for 150 miles. Just thought Id throw that out there since code comes on within 10 miles. Gas mileage is still bad, get the occasional hint of rotten egg smell.

Can you tell if a cat is bad just by looking at the inside of it (that's what mechanic did)

I almost wonder if he was looking inside of the resonators instead of the cats, as the cats are right off the engine and are really hard to get to.

It was for sure the cats. (front and rear drivers side)

At this point I feel like I just need to take it to the dealer...