Hyundai Repair: Sonata 2000 - Ruined Catalytic Converter, aftermarket converters, oxygen sensors

Question
My car lost power about a week ago, I could not get it going above 40m/h. Took it to the mechanic who said that the catalytic converter and the "j pipe"? was completely shot. Gave me a quote for over $4000 to repair which effectively writes off the car. The car has 82k miles on it and apparently if this had happened 2k miles earlier it would have been covered under federal mandate. I get my car serviced regularly, including having a full service at 60k miles. At 62k miles there was a similar problem of losing power which i fixed (replaced sendor p0335 & "phantom" p0740. I am kind of annoyed as you can imagine. Should the mechanic not have been able to tell I had a problem earlier before I burned out the converter?

Answer
Based on the information you've given, there's no reason to expect anyone to have known that there would be a problem with the converter.  Typically, the only thing that would actually damage a converter would be a misfire.  Other than that, if a converter failed, it was most likely a problem occurring within the converter itself.

Your engine control module (ECM) monitors the front converter efficiency while you're driving the vehicle by comparing the output of the front and rear oxygen sensors.  If it detects an efficiency that's lower than the specified value, it'll illuminate the check engine lamp with the appropriate code stored (usually P0420 or P0422).  The computer will be unable, however to detect a problem with the rear converter, which is part of the pipe.  I suspect this is the one you need.

$4000 seems a little high.  Shop around for price for the repairs you need.  Most aftermarket converters don't compare well in quality or effectiveness to the factory converters, but they're also typically much less expensive.

Just as a check, I ran the numbers to check the dealer list price for the converters and front pipe (parts only) for a 2000 Sonata four cylinder and came up with this:

Front converter: $636.37
Front pipe and rear converter: $820 or $1202, depending on which emissions system you have.  

Taking the worst case scenario, using pricing for an area that would mark parts up beyond list and assuming you need both converters, you'd be looking at a maximum of about $2200 in parts.  That still leaves about $1800 for labor to get to $4000.  Trust me that there is no exhaust work that can possibly be done on your car that it could be worth $1800.  For the shop that quoted $4000, you should find out where that $4000 is going.  How much for parts-- have them break it down to each major part if necessary-- and how much for labor.