P0420 / 0430 - Advice Needed

[PSUEng]

Hello Everyone,

I don't post much here like I used to post on the older M3F before its unfortunate demise, and I don't mess with the car so much anymore due to other commitments. But, I'm hoping some members here may offer some guidance on how to handle some pesky P0420/0430 codes I've been getting lately, so here goes the story for your consideration:

CAR: 2003 M3 Coupe 6sp, bone stock motor and software; 80,100 miles, I've owned the car since 2007 and approx. 27,000 miles--no codes before this for anything. I don't drive it much, maybe 2000 miles per year but when I do, it gets plenty warm and I try to give it a good workout, but it's never been on the track or abused by me. The car operates just fine otherwise, and has been well maintained.

I had the car into the dealer back in April of 2020 (76,700miles) for airbag recall and they re-flashed the DME with an updated version per SIB 12-22-08 "M3 S54 Reprogramming for Catalyst". I mention this because only after this did I start getting these codes (around 79,000 miles last summer? I don't remember). I've not studied in depth what this re-flash actually was supposed to "fix", so I'm not claiming it to be the cause of the problem--just noting that I had no problems until this point for sake of argument and anyone who may know if it could be the cause. I had deliberately not taken it to the dealer for this because I didn't want the car messed with, and when they did this without asking me let's just say I was a bit pissed, to say the least.

So fast forward to summer 2021, I get these 0420/0430 codes and I run some Cataclean through the car and they cleared on their own; now, they are back.

I plan on scanning the O2 sensor performance in a week or two as I've seen some others claim that it could be bad O2s, and watching but the only way to really know is observe their voltage readings in real time.

I'm looking at selling the car in the near term and would like to know the best way to confirm that the cats are bad (or good) and how best (economically) to address the problem, if it really is the cats. Fix it (euro setup, re-manned US spec, etc?) Sell it disclosing the codes (is that more detrimental to pricing than sorting it out)?

Any advice on troubleshooting, possible repairs, is greatly appreciated. I've searched around and I don't find many with truly failed cats at this mileage, but I know a lot of guys have header mods anyway, so maybe it's not widespread.

Thanks!

[sbay]

Hey, from the bit of quick searching I was able to do, I found some causes: O2 sensors, bad/inactive cats, and thermostat (car not getting up to temp, not fuel efficient, runs rich, throws codes). I know you said you are getting your O2 sensors scanned, so that’s good. I’m personally not convinced on the thermostat, but I guess check if the car gets up to temp. The link I’ll provide below, where I’m basing this info from, did mention taking the car for a long drive to “re-activate” the cats and “cook” any deposits off the cats/O2 sensors, which I think may be relevant as your car isn’t driven much. Plus, that’s one heck of an excuse to take the car out!

Link to thread so you can research as well:

1. https://www.e46fanatics.com/threads/...-cats.1050205/

I hope this helps, good luck and keep us posted!

[Arith2]

I find it a bit weird that those codes are supposed to be directly addressed by that recall.

[sapote]

I find it a bit weird that those codes are supposed to be directly addressed by that recall.

Agreed. I would blame the dealer for this but I also wouldn't bring the car back to them for more headache.

OP, run a log using ODB Fusion to capture O2 post cat voltage on both banks and this will confirm if they are bad or good.

[Arith2]

Agreed. I would blame the dealer for this but I also wouldn't bring the car back to them for more headache.

OP, run a log using ODB Fusion to capture O2 post cat voltage on both banks and this will confirm if they are bad or good.

The software deletes this problem if it's erronious so there may actually be something going on.

You can tell them that the codes the recall was supposed to fix are now a problem and get a free diag out of it. Then you can just deny whatever they quote and do it yourself. You win with not that much effort. Heck, they may even give you a loaner car. What your car is doing is literally what the recall is supposed to remedy and it clearly didn't. There a special to actually test the cat efficiency. They may have it.

[PSUEng]

Thank you all for the input; I'll start with the O2s. I also wonder how much of this is just due to not driving the car so much. I'll update here with new information.

[Shonky]

Is it possible they've put the wrong (or older) version in somehow hence introducing this problem?