@chucktobias Thank you !! Both are original and still good. I actually tried to add a spacer on the rare O2 sensor, but eventually it caused running lean. I already removed it. The car currently running smoothly, the only thing annoying is the check engine light which indicates a faulty code P0420. If I just live with it, I'm wondering what the consequence will be.

@carlyle705 The only consequence to the P0420 code is that the car won't pass inspection if you're in an area that does vehicle inspections. Also since the check engine light is already on you won't be alerted to other failures.

@chucktobias Got it, thank you again !!
We don't have any vehicle inspection in our area. For the check engine light, I can hook up my OBDLink and check every week. We don't drive it every day any ways.

I thought the ECU will adjust the fuel injection amount based on both O2 sensor reading. At this point, I would rather just live with it. The cost of replacing it with OEM part will cost $5000 CAD, but the car only worth $4500 - $5000, not worth it at all.

Now I have 2 cats, one original, and one aftermarket. I used to switch them back and forth. For the original one, currently I will get the code pending after about 20 minutes driving on city road. For the after market one, I have to drive on highway to get the pending code, than a few laps later, the light will come up.

My question is, if I only run in the city, will the light eventually comes up?

@carlyle705 The ECU calculates fuel delivery using only the upstream sensor. The only purpose the downstream O2 sensor serves is to monitor catalyst efficiency. Pending codes become active codes and trigger the check engine light when the discrepancy from the normal, expected value becomes great enough.

@chucktobias Thank you !!