My intro into this site was prompted by the P0420 on my 2000 Camry 4cyl.
I had replaced the second catalytic converter (which includes the flex pipe) and the code popped up about a month or 2 later. The car has 95K miles on it.
Thanks to the information provided in this forum, I replaced the upstream O2 sensor. Because it was the easiest one. I do the easiest/cheapest replacement first. Unfortunately the code came back. It would always come back, even after multiple resets. I started to question the cat, which is manufactured by Walker.
I was so desperate, I even called 2 local Toyota dealerships and their service guys were adamant that the problem was caused by/due to the aftermarket cat. I have dealt with Walker exhaust for over 30 years and have never been disappointed.
I finally spent the money and bought the downstream o2 sensor from Toyota. It ended up being cheaper than Advanced Auto Parts' replacement. It has been almost a month and the code has not returned.
I can't speak to other aftermarket exhaust replacement cats, but it's for sure the Walker cat worked for me.
I hope this helps someone else who is in the same predicament I was.

 

sound good!I can't wait to take a look at your car, can you share some pics?

 

I must've jinxed myself.
No sooner did I post this message, this afternoon the CEL illuminated itself. I just read the 2 codes. 1 P0420 and 1 P0420 pending.

I really thought it was fixed otherwise I would not have posted.

To redeem myself in this forum I'll find the fix and post it.

My apologies to all.

 

I really don't understand your logic in replacing parts and see if code goes out. You need to troubleshoot and then replace most probable parts. So far you have spent a lot of money and same code has come back on.

fixing check engine lite is not easy and you sometimes need to have proper electronic tools that dealer have to troubleshoot the different sensors. Sometimes it might be cheaper to let dealers handle it or independent pro senior tech to do it... Some dealers are letting rookie do replace until fixed theory repair--stay away from those--cost you lots of cash... Find a really senior expert tech who troubleshoot instead of replacing parts needlessly... sometimes it could be simple as a crack in vacuum hose, wiring problems...

what makes you think its not the first cat converter...

I have a 96 camry 4 cylinder and check engine lite came on for cat(most probably) but have not started repair yet as parts are expensive and cannot afford to second guess myself. I will be doing lot of web research on it,,,but so far looking big guess its first cat converter and dealer wants $200 for it for part only.--easy to replace.

 

Downstream cats are only for emission control. They are not actually monitored by the ECU. Your air fuel and O2 sensors just compare readings between themselves to see if the cat is functioning. If there is no sensor past the cat, then it cannot check it efficiency. Very rarily will downstream cats need replacement unless physically damaged.