If there is a failed O2 sensor the scanner should tell you which one. There are sensors located 2 pre cats and 2 after cats. The front ones give information to the computer to set the mixture, and they will vary for Rich to Lean quite rapidly. The back sensors should remain fairly steady, to show that the Cats are doing their job. Bank one sensor will be on the side of the engine that the #1 cylinder is located, and it should be the passenger side. Bank 2 should be the drivers side. Sensors are listed as Bank 1 sensor 1, bank 1 sensor 2, bank 2 sensor 1, and bank 2 sensor 2.
If you are not getting a "Check Engine Light", sensors should be working correctly. If you want to see what each sensor is doing, you will need to find someone with a "Diagnostic scanner", which will show what each sensor is doing.
