Cat Replacement Options for B8 RS5

Hey everyone, names Steve, long time follower of the rs5 forums. This is my first time starting a new topic, so hopefully not screwing anything up!

Long story short, there are telltale signs the cats are going bad in my 2015 RS5 with only 55k miles on it. I have JHM downpipes and rest of exhaust is stock. Bad news is I am just outside the 8 year emissions warranty, so SOL there.

I am just looking for any alternatives anyone has seen for replacing the cats. I have looked into JHM and am considering their de-cat racing headers, but truthfully do not want to run catless. Aside from OEM, has anyone seen any other solutions? Especially interested in high flow options if available.

I did see in other forums that there was discussion on going with JHM racing headers and running downstream cats, but never saw any posts on anyone actually doing it. Does anyone have any insight on this / has done this to their car already?

Thanks everyone!

I know an rs5 owner named Steve. Any chance you’re from Atlanta?

Anyways hit up JHM as they make headers and if you’re lucky they’ll maybe sell you a stock cat replacement.

55k is about right for RS5 Cats. Anyone else running their RS5 with more miles than that has damaged Cats whether they know it or not. An inspection from the S1 sensor bung with a video inspection camera will be the best way to inspect for damage. As for what to do if you want to keep Cats, depending on where you live and what you have for good shops around?? A downstream Cat is best for both the RS5 (stock or modded) and even the 3.0t (last ok if you’re stock, but not Stg 1 or 2). For OEM Audi put the Cats as close to the engine to speed warmup which helps in the EPA startup emissions cycle, but that extra heat that helps the warmup cycle damages the Cats. I’m going to go with a Downstream Cat on my S5 but I think I’m leaving my RS5 Catless, the Motronic does a pretty good job of keeping the raw fuel smell low on the RS5 compared to my older C5 RS6 that’s Catless.

Interesting topic, and with about 90k KM on my car, or about 55k miles, it looks like I better pay attention to this topic! As we say here… “subscribed”… I already had a very light “tinkling” sound from the left side cat area and my mechanic said “yeah, you are right there is likely something loose in there but just ignore it for now”. I guess I have some work to plan for in the next year or so!

If you’re lucky it’ll break up into smaller chunks and end up in the Resonator, Muffler, or completely blow out the back. Evenutually anything in the Resonator or Muffler will probably rattle around and erorde enough to go out the back. Unlucky and large chunks will break and block the flow, causing high EGT’s, driveability issues, an O2 code and more.

No, but I did almost buy my RS5 from Atlanta! Flew there from Chicago, car was falsely advertised as being “pristine” and looked great in pictures, but got there and it was a POS. Paint looked like it had hardly any clear coat on it and they tried to pull the whole “it’s just dirty, we will wash it while you are signing papers” trick. I swore I would never try to buy a car from Atlanta again, live and learn!

Anyways back on topic. I did reach out to JHM and they only offer cat deletes ATM, so no luck there. I also reached out to Magnaflow since they have something “in development” but have no ETA, so I don’t have my hopes up there.

Also good to know downstream cats are feasible, just didn’t know if anyone on here has done it yet. I will reach out to a few exhaust shops and get some pricing / options to see what this will look like.

Mine started as a somewhat metallic rattle sound on the right side towards the back of my car, which leads me to believe it’s chunks of the cat. Only hear it under certain rpms and went through the exhaust to make sure everything was tight. I initially thought it was the exhaust valves, but after reading here I’m guessing it was the cat breaking down!

Does anyone have recommendations for high flow cats they have used before?

I’m surprised to hear that about JHM. I know when they first started modifying the Headers to Catless they were originally planning to offer high flow Cats as an option.

According to this a5oc post it is possible to pass the an exhausts inspection

I don’t imagine you will see any of the big companies develop anything for the RS5 as it’s just such a small market. Will have to do a custom job. Manufacturers have been trying to get their Cats closer and closer to the engine, especially the performance cars (dirtier emissions) as the Cat’s heat up faster for the Cold start cycle the EPA runs. Downstream Cats should be fine for emissions tests with any other place as you just warm your Car/Cats up well before you take it to the emissions place. If it’s a short drive there take the long route and get them nice and warm which improves their efficiency. Bad side of putting them up so close to the engine is the hottter and stronger gases cause faster degredation of the Cats.

Just a heads up, if you have the JHM downpipe/xpipe combo, there’s now an option to add cats after the downpipes. It eliminates the X but if you blow your stock cats and run headers, there’s now a way to do so easily. Just reach out,

Thank you for providing that information! I have under 60k miles on my RS5, without any of the “broken” powdery catalyst material others have seen when the catalysts break down and bits get pushed toward the tailpipes. However I do have a very faint “tinkle” sound heard from the left front wheel well with engine idling. My mechanic simple said “good ear!” but not to worry about it now as any action is a big job. But if replacing stock cats now have a possible emissions-compliant (or attempting such…) that is good to hear!

If you don’t have to pass emmissions I wouldn’t worry about it if you and your mechanic are keeping an eye on it but I can almost guarantee you if he pulls the S1 O2 sensor out of both Cats and looks with a borescope or video inspection camera he will see damage in both Cats. I’ve seen the damage in RS5 Cats starting prior to 30k miles.
Hopefully small pieces will break off and get knocked around and as they work there way to the back over time be not much more than a powder, but occasionally large pieces can break off and cause restrictiosn which can cause a spike in the EGT on that bank and loss of performance but most likely throw a Catalyst code fairly quickly.

This is true. I’ve looked at A LOT of RS5 catalysts and they tend to erode on the side closest to the exhaust port, with the erosion slowly working backwards. I suspect it’s caused by fueling issues which get left unattended or don’t make themselves known.

I have 85K on mine and I did bore scope them about 10K miles ago. One had very mild erosion the other did not.

I have a set of headers in now and will be inspecting the cats some time soon. I’ll post a few photos up. I know one has a bad catalyst as the owner’s car threw a catalyst inefficiency code. I have another local client who’s about to do headers and the JHM downpipes with catalysts.

The 3.0T cars, whether S4/S5/A6/A7/etc… have the same issue. There Cats are in about the same place but at least the Cats are bolted to the headers instead of welded as one piece as in the RS5’s. And in most versions the Right Cats can be removed in place and the Left Cats only require the enigne/Tranny to be dropped around 3~4". Many 3.0t engines have been destroyed because of bad Cats. I think Audi’s tight Cat/header design may have helped them with quick startup of the Cats for emissions and definitely a theft deterrent but the cheap Cats won’t last for sure.