Q: I am getting the code PO420 Catalyst efficency low bank on 2008 Chevrolet Aveo

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What is the likelyhood that the catalytic converter has indeed failed or could I get that same code if it were just a sensor gone bad?
(3) Answers
this is an obd2 trouble code table, it can help you understand the meaning of code.
It means that your muffler system as well as your exhaust system need to be replaced. If it is the catalytic converter, you are looking at close to $1,000 or more to get that replaced. Take it to Midas or to Merchant Tire Auto or to your dealership or to a trusted mechanic to have them look at it. Have them run a diagnostic on the Check Engine light to see if that can tell you what the problem is. More than likely it could be the exhaust system going out.
Wow , I can tell you are concerned , it may not be that bad.
If you go to a local library that has a data base ( all data or Chilton) it will give you the complete description of the code , what it means , how to address it.
At the very least you will be able to determine what is not plausible in regards to the code.
GM's apparent normal strategy is to compare the pre catalyst oxygen sensor which produces a varying rich or lean signal, much faster than you could see , versus the post catalyst senor reading.
The post catalyst senor reading is not suppose to vary as much as the sensor before the catalyst.
Because the catalytic converter is oxidizing the remnant HC ,and CO pollutants , so any changes are slower to result.
In short since the two are being compared , if one is not correct all bets are off in the analysis being correct.
This is true to the point of heated o2 sensor not functioning as designed, in other words a blown fuse to an o2 sensor.
I usually subject the front sensor to scrutiny , that it is functioning up to excellence par , a contaminated rear senor may be overactive , a recently replaced rear could possibly have the same result.
In short this line of investigation should be exhausted before turning to the exhaust proper as new exhaust parts not needed will also be damaged , possibly by incorrect or inoperative emissions sensors.
Just a thought,,yep you are right.