2002 Pontiac Bonneville with 172k miles, automatic transmission. Code P0420. Following is printout of code from Autozone. Begin--Definition Catalyst efficiency low-bank 1 Explanation The oxygen sensors monitor the Catalytic Converters ability to store oxygen. Probable cause 1 .-Catalytic converter defective (failure possibly due to #2,3 or 4) 2.- Engine misfiring or running condition 3.-large vacuum leak 4.-Engine oil leakage into exhaust-valve guide seals, piston rings.--End My thoughts-I do not think it is 2-car runs fine. It is my wife's car, but I drive it occasionally. I just drove it, and it ran fine. No misfirings that I can tell. Just asked wife--she said it runs fine, too. No misfirings that she can tell. 3-do not hear any large vacuum leaks. 4. car exhaust is not blue-it is clear. My main question-could it possibly be one of the oxygen sensors instead of the catalytic converter? Scotty recommends the Ancel AD410 enhanced OBD II. Will this help me narrow down what it might be? If not, is there one that is under $100 that would help me narrow it down? I have replaced head gaskets on two different cars. No parts left over. Both started right up.
Before getting to the conclusion of bad catalytic converter, you should check the back pressure:
It certainly could be the O2 sensor, but you need to go through the troubleshooting and confirm or rule out. This site (also in FAQ) helps me out a lot on how to go about troubleshooting these codes:
https://www.obd-codes.com/p0420
As far as OBDII scan tool under $100 that Scotty recommends, they can all read these Check Engine codes and give you live data on engine parameters: coolant temp, O2 sensor data, misfires, etc all stuff you need to know for troubleshooting some of these codes. It’s worth the investment to get one. Generally the higher the price the more capabilities it has. Also, make sure your vehicle is covered before you get that particular tool.