Toyota Repair: Check engine light, lifespan of a cat, toyota 4runner

Question
Thanks for that explanation and advice.  With regards to the 2nd question - is 70k the usual lifespan of a cat? and what factors could shorten the lifespan of one?


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1999 Toyota 4Runner.  Check engine light on.  Brought to local garage - code P0420 came up.  They said catalytic converter needs replacement - est $1100 repair.  A year ago check engine light came on and an O2 sensor was replaced.  Two questions 1 - How do you know its the catalytic converter that needs replacement vs. the sensors 2 - Should catalytic converter be going at 70,000 mi?  If not, what is this happening.
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They are correct in the diagnosis of needing a catalytic converter. I would try to get an aftermarket one instead. they are closer to 100 bucks instead of that 1,100.  The computer monitors an O2 sensor both before and after the cat. If the Cat is working correctly the front O2 will cycle normal, but the rear O2 will hold steady. When the converter fails the exhaust passes freely through it and the rear O2 sensor starts to mimic the front one indicating the Cat is not doing its job.

Answer
Yes, I would agree that 70K is a short life span for a converter.  What usually kills them is getting raw fuel dumped into the exhaust from a misfire. However, Toyota takes great safeguards to prevent this by shutting off the injector when there is no spark. Otherwise I really have no explanation to why it failed so soon.