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