B7 S4 4.2 v8 Blocked Cats? Any help or ideas?

Over the past year or so, I’d get a Warm Up Cat MIL on once in a while. P0431 Mill ON. I’d reset it, and carry on my merry way. But then it started getting more frequent, and eventually it was both Warm Up Cats, and pretty much coming on 20miles after I reset it. P0431 Mill ON and P0421 Intermittent.

Now here’s the thing, my S4 is an automatic with miltek sport downpipes (fitted way back in the day before I bought the car), and has at least a stage 1 remap. I know all the info about aftermarket downpipes, and just putting a cat where the pre cat is and doing away with the main cat. This therefore requires coding/remapping to remove the o2 sensor faults. These never appeared before, so I assume that the remap has removed these faults when the downpipes were fitted. What I find odd is, how can the car now think that the precats are blocked, when surely it should of been coded out?

So sorry this is all a bit long winded, but I’m sooooo stuck as to what to try first. Surely both o2 sensors cant of gone at the same time? And which ones would I change if that were the case lol? I’ve got VCDS, and think the sensors are working correctly, but I dont really know what to log to be certain!

Also, if I reset the MIL, the car “seems” to run better, and the engine note definitely changes. Now, what does this suggest, if anything? Simply a compensating fuel mixture thing because of the MIL, or that the o2’s are breaking down or something?

It’s all a big faff really. I’d just knock the pre cat out to be sure, but seeing as I’ve got miltek downpipes I cant do that. The only option I have is to buy a brand new pair of OEM exhausts, and gut the precats…and/or change the o2 sensors, which is all a LOT of money lol, so if I can have some idea of whats going on first, I’d feel much happier!!

Thanks for the patience, and any help the guru’s can give :slight_smile:

Man that’s a tough one.

There are a lots of it depends.

First off. The miltek cats were known to fail over time so its very possible that they have gone dead.

The P0431 is for what you would see with a bad cat.

But then it also depends on the tune as well. Some of the B6 S4 tunes were kinda garbage. This is why almost everyone went JHM tuned so they wouldn’t run into these issues.

I think the only way your going to know is to pull the Dps. Because if the cats are bad broken or clogged its going to cause many more issues sooner or later.

Thanks for the reply.

Yeh, I’m at the stage that I’ve got no option but to pull/change the DPs, I guess I just wanted some reassurance/sympathy haha.

Have you ever known something like this to be both o2 sensors? I cant see it myself, as it’d be very unlikely/unlucky.

Tune wise, I’m in the UK, so JHM isnt, or certainly wasnt an option back then as far as I know, but the tune “seems” ok, except the exhaust tips do get rather sooty.

And yeh, its been getting worse for a year or so (I dont drive it much anymore because of it) so I’m super worried of anymore damage it might be doing too.

Thanks again for the help, if you think the only next option is to drop the DPs, then thats good enough for me!

I would say it is super odd that it would be both sides. Its not completely out of the question but it seems odd. The only other possible thing I can think of at this time would be that there are bad 02s but generally they don’t fail with that code.

Just as a final last check, what could I log/read in VCDS that would prove the o2s are working correctly? Dont get me wrong, I’m sure it must be the dps, but if its easy to check the o2s are working, then it wont do any harm.

Cheers again for all the info/help