Before replacing a cat I'd verify 3 things; 1 - make sure there are no obvious leaks within the exhaust system, especially in front of the cat. 2 - make sure your ignition system (spark plugs and coil) are in good condition. 3 - verify your O2 sensors are good. Someone else might come along with something totally different to say, but that is how I go about catalytic codes.
Just for example, years ago I had a girl bring me a Mitsubishi that threw a catalytic code, so she took it to an exhaust shop and they told her the cat was bad, we'll fix it for $900. She let them do the work and the code came back a week later. The shop told her some BS, and that they needed to replace the cat again, she just had to pay labor cost or something to the tune of a few hundred $ more. I talked her into letting me look at it first and easily figured out her spark plugs needed to be changed. 4 new plugs later, and she was good to go.
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