My sister had one of these codes, that her catalytic converter was below sensor level, though she recently passed emissions.  If she runs premium gas would that help?  Does any product help with these?  

 

The lady at Auto Zone after finding my cat. code came up on a 99 Outback with a 2.2 said probably the misfire or other less than optimum combustion triggered it.  Is that right?

If the spark plugs are old that can definitely cause this code. A good tune-up is often able to make it go away.

 

Plenty of other things can cause the P0420 code as well, but a tune-up is the place to start if it hasn't had one recently. 

Also look for a hole or leak in the exhaust before the second O2 sensor.

1. post the code.  probably an 0420 but best to be clear

2. nearly anything can cause an 0420

 

start with anything triggering the CEL (other than 0420) or causing poor gas mileage, exhaust leaks.

 

simplest fix is an O2 extender - works often enough it's worth the $5 to never think about it again.