I have a stock '06 6mt at 72k miles that's gone through two sets of cats in a year now. I replaced the o2 sensor with the cats the first time.

From what i've read running rich or incomplete combustion can foul up the cats quickly so i'm planning on replacing the fuel pressure regulator and the spark plugs.

Am i on the right track here? Search only turned up a bunch of HFC's

 

That's what you need. HFC or TestPipes. Stock Cats suck, will eventually fail & restrict your engines breathing.

 

There is something going on causing the cats to fail, misfire, extreme rich condition etc. I suggest fixing that issue first. Any codes? How are you diagnosing that the cats have "failed"?

-Rob