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HiCustomerand welcome to Just Answer! The code you are referring to is P0420 Catalytic Converter Below Threshold of Efficiency. Which means the computer for the car is looking at the oxygen sensors in the exhaust, you have at least one before the catalytic converter and one after. If the computer does not a specific difference between the first and second oxygen sensor it will set this code.
Most times this code is set because the catalytic converter is bad, but it can also be set by a bad oxygen sensor, an exhaust leak, or many other running problems. 90% of the time a cataylic converter will fix the problem if the car has alot of miles on it with nothing else being done.
If you go to the dealer the converter will be much higher in cost, but there are small muffler shops that do it at low price in comparison. I just has to put one on a Chevy Blazer two days ago, and the factory converter was $525.00 and a local muffler shop put it on for $160.00. Basically you need to call around and get your best price.
If you need any further help please ask.