Intermittent P0421/P0431 on 575m

Hi folks, I've been getting intermittent P0421/P0431 and I want to try and get to the bottom of it. I've been clearing it whenever it popped up and living with it since car is running just fine. So last month I had a slowdown light and my mechanic replaced the cat temp sensor controllers. We were surprised to see original controller with 2002 date codes. Plus my car had 61k miles now so they held up quite well. Anyhow, our plan is to replace the O2 sensors next if the check engine light pops up. I just want to check with you all to see if you folks had similar issue before and what the usual culprit is. Worst case, if the codes come back after chaining the O2 sensors, i'll probably change the CATs with high flow ones from larini or something. Any other possible causes?

P.S. one thing is, when the codes come on, both sides come on together. Is it likely that both sides crap out at the same time? Or one bank automatically triggers the other by design? Or they are independent and should come on separately. If they are independent, i'm thinking maybe something up stream like air injection pump or something crapping out during cold starts etc.

 

Thanks for the replies gents. I'll try and talk to my mechanic as well and see if he can check things visually. I'm not sure how often the pre-cats fail. If main cats i think it's probably easier and cheaper to replace if I go aftermarket route. For pre-cats I am not sure what options i have. Especially in California... maybe time to register the car in Montana and go cat-less .. Thanks again gents!

 

Thanks for the info again Taz.. I will put the on the swap out list. What’s interesting is that when the check engine comes on it comes on for both banks at the same time. So maybe it’s more up stream? Anything that might trigger the codes on both sides at the same time?