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?

 

Hi Folks, just want to follow up on this topic from my end.. it's been a while and the code kept on going on and off. When I started this thread the car had about 61k miles and now it's inching towards 81k miles. And the code kept on popping up and getting more and more frequent. Since I first posted, I have done the following (for the engine ):

1. Major service (usual belts, new plugs and filters etc.)
2. New fuel pumps and filters
3. New O2 sensors, all 8x of them
4. New secondary air pump and relay
5. I already have new thermocouples and the control units
6. Inspected the CATs with borescope and looks to be in tact but functionality might be a different story
7. Ran cataclean a few times but plan to do a couple of more runs
8. Take out the ECU and clean the connectors etc. just to be sure (they were clean and no corrosion to start with)
9. Checked the functionality of the secondary injection valves (4x on top of the manifolds) and they are confirmed working fine

So with all the above, I am still getting the same P0421/P0431 (Warm Up Catalyst Efficiency is below threshold) on both banks at the same time. So starting to think that the CATs are loosing their efficiency. Since I'm in California my options are limited and even if I want to buy the OEM parts, parts are limited or NLA. And I have no aftermarket options to go with. Fabspeed does some re-core'ing but I don't see them list 575s so I pinged them but have low to no hope. So, before I register the car in Montana or something, want to give it a one last shot and ask the collective mind of the internet and see if anybody has any thoughts or suggestions.

I've gone through every single system and component on this car and spent almost as much as I paid for the car lol. So yeah, hoping I can find a solution to keep it in Cali but chances are getting slimmer and slimmer. And I don't want to just sell it to out of state. From experience, looks like every system starts to fail around 75k-80k miles..ask me how I know . Anyhow, thanks in advance folks.

-Han

 

Unfortunately they were the first one I replaced along with the controllers but I will double check with my mechanic to be 100%.

 

Actually I was going through my service records and looks like we just replaced the thermocouple ECUs and not the thermocouples. So I'm gonna try and swap them out as last ditch effort. Pray to the Italian car gods for me lol.