Codes: P0135 and P0420 on I30 2001 Infiniti I30

[lynnjester]

Service engine soon light came on (reset and it returns)
Codes: P0135 and P0420

Reading device said it is the O2 sensor Bank 1 Sensor 1. I know sensor 1 is before the cat convertor, but do not know if it is on the side by the firewall (I think this is bank 1 as it is on my toyota) or on the side by the radiator. Please advise if you can.

I understand that code P0420 is a cat Convertor trouble. Hopefully it is just the bank 2 sensor on the convertor or downstream from the convertor. The car has 180,000 miles so maybe replacing the original spark plugs would help. They appear to be burning nice (tan color).

Any advice on this one?
L. Jester