What caused my catalytic converter to go bad?

#1
Posting in here to get an idea of why my CAT might have gone bad. I've got an automatic 2006 CE with 111k miles. The car passed smog on March 5th. Just bought the car off Craigslist on March 10th, and the CEL came on after driving it about 85 miles. Seller is ducking my calls, so no luck there. Took it to O'Reilly to get the code read (pictures attached). Then took it to a mechanic who told me the 02 sensors were both reading at variable wavelengths and recommended that both catalytic converters be replaced. He didn't tell me anything about what might have caused the CAT to go bad in the first place, even when I asked. I know they don't just deteriorate out of nowhere, so I want to find out and fix what ruined it before I get a new one.

I don't really have any symptoms besides the CEL, as far as I can tell. Gas milage is not far off from normal (26mpg), there's no sulphur smell, not idling rough, can reach and maintain speed of 80mph, doesn't have trouble shifting gears or accelerating up hills, starts perfectly fine every time, doesn't misfire.

Please help! I don't want to rack up bills going to mechanics replacing a bunch of parts, as I know catalytic converters can deteriorate for many reasons. Thanks for reading! Let me know if there's any other lucrative information I might have left out on accident. IMG_20180313_171423027_HDR.jpg IMG_20180313_171605343_HDR.jpg
 
#2
Heat, weather, erosion and time will cause any catalytic converter to go bad. If you have an full after-market exhaust setup, you WILL get codes (this from my personal experience).

oh boy.... do u know how to wield?
 
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