Cats...when they go bad.

[lodit]

Just had a check engine light go on.

The Uptown Markham Audi dealership did a code check.

Bad cat, bank 2.

Replacement cost in CAD for ONE catalytic converter is $1360 for parts, $500 labor.
Cost for two is basically parts added.

Now, I ask: replace both cats or is it time for aftermarket exhaust?

Thinking Milltek or maybe custom Magnaflow.

Also, had a little fender bender at a gas station. Some dude reversed and hit my driver's side rear bumper. Slight scrapes, can't see any cracks, but scrapes are obvious enough...

Question is: do i go for a full bumper replacement along with the parktronic sensors? Parktronic doesn't work currently due to a faulty sensor. Or do I simply ask for a repaint of the damaged area at a local body shop? They seem like nice people, so I don't want to kill their insurance, but my bumper is crying because of the discoloration it suffered. Also, it gives me a reason to get the trunk painted at the same time and repair the other scratches on the bumper. What's your take?

[Affinitive]

Before you fork out the big bucks for for cats, consider this,.....
To this day I have not seen aftermarket cats work without setting fault codes without doing a custom programming on the ECM. Audi's and VW's are well known to have the most sensitive O2 and cat efficiency monitoring strategies there is. Basicly that also means that these newer vehicles will constantly self test the O2 sensors and cat effiency more precisely than any tech would ever be able to do. Basicly when car says the O2's and cats are failing, the car is right. That said, there are a few models, your specific Allroad being one of them, that needed a software update for the ECM because the the monitoring strategy was too sensitive. Yes, the cats are starting to show some age and begining to loose some of their efficiency but are probably still well within acceptable and federal legal limits if the update programming has not been done (reflash).

When it comes to VW's and Audi's, aftermarket cats are a big big no no unless you're going way extreme in performance. Even with mild to moderate performance, you're still best off with just OE cats.

Cats and O2's are always to be done in corrosponding pairs. Typically when the cats do start to fail, you wana do at least the primary O2's as well to save money down the road in terms of labor, especially if this was a 3.0L. O2's on them are a real pain. O2's are 80 - 100K throw away parts anyways.

The way Cat efficiency faults are properly diagnosed on these cars is to varify the need for software updates, which your specific AllRoad does have a software update available to specificly address this fault, and upload the update and reset the fault codes. When there are no updates available to cure cat efficiency faults, then reset the fault and wait for it to reappear. If it reappears with no other faults, then it's time to do tha cats. Again, always in pairs, otherwise it's guaranteed to bite you later.

In california, there should be a couple of independent shop with cerified VAS 5052's. Contact them and have them find the appropriate TSB to get the programming CD part# so they can order it. If you can't find an indy with the factory scanner, then you're stuck with the dealer for the for the update.

Like I mentioned, for Allroads, it was 2001 only for the cat efficiency update. If it does need cats, then go OE only unless you have VERY BIG plans for performance. Aftermarket exhaust from the cats back is another story though, you can do whatever you want there.

Though if you plan on being up here in WA at any time, I can handle the reflash for you :)

C

[oc-drop]

my 2cent reply to the previous post is that I have Magnaflow cats and have thrown one cel in the past 3 years (lasted 2 days the week after my install)... no faults since

and to lodit, I highly reccomend a new performance exhaust... it'll sound like a whole new car you can fall in love with all over again =)

[skiroad'r]

how many miles on your car? Any chance they are covered by the Federaql Emmissions Warranty? Most dealers selectively remember this little tidbit.

[Affinitive]

Whoa -
Excuse me, for some reason I thought you had an Allroad. Doesn't really matter though, all 2001 APB 2.7T's would need the reflash,...eventually.

Sorry for misreading there.

C

[lodit]

Thanks for all the info.

Here's the situation:

2001 A6 4.2 Quattro Sedan w/ Sport Suspension
139k on the odo, runs great.

CEL came on about 2 weeks ago.
Went to one Audi dealership to have it serviced and oil change.
Says I need new inner tie rods because of vibrations. I said, no thanks for now.
Fault code returned Bank 1 O2.
Codes were reset, cleared and car road tested. No issues they said. Got car on Thursday.
Next day, CEL came back on.
Went to another Audi dealership on Saturday.
Says tie rods are fine, vibrations through steering caused by hubcentric rings inside wheel not fitting properly. I said, "huh, what?" They temp fixed by thickening the hubcentric adapter rings inside the wheels. Reduced vibrations somewhat, but not completely. Thinking maybe it is tie rods after all. If so, do I go OEM or other? I just had the outer tie rods replaced a few months ago with OEM parts. All control arms and tie rods were replaced Oct 2006. But I know these wear pretty quickly on this model. Is Meyle or Lemforder any better than OEM for these particular parts?
Fault code returned Bank 2 Cat. Says bad cat, needs replacement.

What gives? One dealership gave me a code for Bank 1, another gives Bank 2?

From what you guys have stated, is that I should see if a reflash will eliminate the recurrence of the CEL for a sensitive O2/Cat sensor.

If it has been done, and the car is still returning the CEL fault code...then time for new cats?

Also, you're saying I should stick with OEM cats instead of aftermarket, unless I aim to get the ECU recoded for them?

If it turns out I need new cats, then I'd probably be better off going full out aftermarket, in terms of price/performance wouldn't I?

Or can I go to a junkyard and get used cats from the same model vehicle?

I'm hoping the dealership can resolve these CEL code faults.

I'm currently in Cleveland, OH for work. Won't be back in CA for a while.

So, thanks for the offer to reflash the programming, but WA is quite the trek. :)

[xpl0sive]

Thanks for all the info.

Here's the situation:

2001 A6 4.2 Quattro Sedan w/ Sport Suspension
139k on the odo, runs great.

CEL came on about 2 weeks ago.
Went to one Audi dealership to have it serviced and oil change.
Says I need new inner tie rods because of vibrations. I said, no thanks for now.
Fault code returned Bank 1 O2.
Codes were reset, cleared and car road tested. No issues they said. Got car on Thursday.
Next day, CEL came back on.
Went to another Audi dealership on Saturday.
Says tie rods are fine, vibrations through steering caused by hubcentric rings inside wheel not fitting properly. I said, "huh, what?" They temp fixed by thickening the hubcentric adapter rings inside the wheels. Reduced vibrations somewhat, but not completely. Thinking maybe it is tie rods after all. If so, do I go OEM or other? I just had the outer tie rods replaced a few months ago with OEM parts. All control arms and tie rods were replaced Oct 2006. But I know these wear pretty quickly on this model. Is Meyle or Lemforder any better than OEM for these particular parts?
Fault code returned Bank 2 Cat. Says bad cat, needs replacement.

What gives? One dealership gave me a code for Bank 1, another gives Bank 2?

From what you guys have stated, is that I should see if a reflash will eliminate the recurrence of the CEL for a sensitive O2/Cat sensor.

If it has been done, and the car is still returning the CEL fault code...then time for new cats?

Also, you're saying I should stick with OEM cats instead of aftermarket, unless I aim to get the ECU recoded for them?

If it turns out I need new cats, then I'd probably be better off going full out aftermarket, in terms of price/performance wouldn't I?

Or can I go to a junkyard and get used cats from the same model vehicle?

I'm hoping the dealership can resolve these CEL code faults.

I'm currently in Cleveland, OH for work. Won't be back in CA for a while.

So, thanks for the offer to reflash the programming, but WA is quite the trek. :)

i don't know about CA, but here, it's illegal for junk yards to sell used cats... my cel comes on once in a while regarding bank 1 o2 or something like that... i'm simply planning on cutting my cats out and doing a O2 sensor "modification"
http://www.audiworld.com/tech/eng53.shtml

but thats just me

[Alpinevelo]

What about the Magnaflows on eBay. They are about $70.00 has anyone used them?

http://cgi.ebay.com/ebaymotors/MAGNA...spagenameZWDVW

Depending where you find them but sometimes they are advertised for Audis from 95-99 and at times it will read that they are Universal...?

[oc-drop]

I'm using magnaflows right now with no problems =)

just make sure the O2s are not bad because they will foul your cats

and as far as new tie rods, meyle is ok (i am using meyle ones for 2+ years now with no problems, but the control arms are Febi which also makes the tie rods, i just happened to keep the meyle tie rods on since i installed them before the control arms) ~ I've seen more shops using Febi than Meyle so I would get the Febi if you can!

[audifirst]

I have as well aftermarket cats for two years and I have no issues, go for it... OEM is nice but way too expensive

[Affinitive]

Ohh woops

I'm sorry again. For some reason I thought or assumed you had the 2.7T. Sorry about that. I read through stuff too quick sometimes and now see it also says 4.2 below the username. Sorry about that.

Sounds like the Cats are definitely done if it's sets faults several times now. As for the O2 sensors, they've already outlived their life expectancy at 140K. Also, again I appologise, there were no updates for 01 4.2's. Sorry , didn't mean to get your hopes up.
As for the vibration, loose tie rod won't really give you a vibration. Sounds like the tires just need to be balanced again. Not unusuall at all. The fact that the vibration changed most likely is because they removed and reinstalled the wheels on a different orientation to the hub. Years ago GM I believe it was had a service buletin about marking wheels to one of the studs when removing so the wheel would be reinstalled to the same orientation to avoid changes in vibration when the vehicle had tires with some wear and tear on them. back in the day they used to rebalance tires once or twice during their life span. Now a days they try to sell you new tires.

About Tie-Rods. They shouldn't need replacement unless the joints are loose. On all these cars I will only replace Tie-Rods as complete assemblies using the S4 Tie-Rod assemblies. Ends up being cheaper as both side can be done in 20 minutes as oppossed to messing around with trying to free up the rusted threads trying to change justthe outers. Much less labor involved and usually ends being the last time the tie-rods ever need to be done when using the better S4 inner&outer assemblies and avoids having to pay for alignments multiple times when chasing bad tie-rods one at a time.
I really don't fancy Meyle parts at all. Some things I will not even consider meyle. Lemfoerder is pretty good stuff. I always keep at least one set of S4 lemfoerder Tie-rod assemblies on hand for my customers.

As for aftermarket Cats. I didn't say it could not work, but I certainly wouldn't hold my breath. If you do find some, I would get a written warranty in hand stating the manufacturer will reimburse you for the parts and labor should they keep setting fault codes. On earlier cars, like 96 - 98,99ish range there would be a little more flexibilty but even the older ones I have had to pull out many many aftermarket cats and replace with OE because faults would keep setting. Again though, if it happens on someone else's dollar, who cares, right. Though keep in mind that cats don't actually restrict performance one bit until you get into these big extreme performance engines. If it was my car, I would go with OE cats and some aftermarket cat back system. Definitely do the O2's as well considering the age. Like I said, the O2's have already outlived their life expectancy. In fact, I'm surprised they have not been an issue before.

C

[oc-drop]

...
As for the vibration, loose tie rod won't really give you a vibration...

About Tie-Rods. They shouldn't need replacement unless the joints are loose.

when my tie rods went bad (loose joints) the car vibrated above 50mph... I got new ones and the vibration went away.

[Affinitive]

Are you sure that the shop putting the tie-rod on didn't conviently happen to rotate the tires as well

I see what you're saying though and we could back and forth for days. Just tellin' what I see having worked on hundreds if not thoudsands of these and other types of cars. Given the right circumstances, you could very well be correct.

C

[lodit]

Ok,

So basically we've narrowed it down to the fact my cats and the O2 sensors have served their life expectancy.

Question: Bad cats do what to your car? Other than throw the check engine light.

Replacement could be OEM cats and aftermarket catback, full OEM, or full aftermarket.

Downside in using aftermarket cats is fault code re-appearance, but programming the ECU should take care of this?

Prohibitive cost for OEM cats is deterring me away from them, but do you have any aftermarket suggestions for cats that would seem price/performance worthwhile?

As for vibrations, it looks as though the hubcentric adapter ring does cause vibration if it sits a little too loosely on the hub, and not snug as it should. Due the fact it's made of plastic, there's never a guarantee a set of four will be uniform, hence vibrations. I'm getting another set of rings sent out to me, and will see if that changes anything.

Please elaborate on what the noticeable symptoms of bad tie-rods are...vibrations have too many factors to consider, can we specifically attribute certain driving characteristics to bad tie-rods?

Thanks for all your help and advice. If only everyone were as accommodating as our AZ community, imagine what life would be like.

:)

Cheers,

Steve

[Affinitive]

Worn out tie-rods generally just give you loose steering and clunking type of noise when working the steering wheel. And of course alignment issues as well with the steering getting loose and sloppy.

As for the little hubcentric gadget, not too sure what to tell ya except that i see plenty of aftermarket wheels without of any kind of centering adaptors and the wheels always end up being centered to the hub when properly mounted. As in the wheels have to be sequentually snugged and torqued to prevent the wheel from getting damaged. Are your wheels damaged???

As for cats, just find some that are known and guaranteed to work without needing custom programming done. I wouldn't hold my breath though or it may just be very hard to find some. Inefficient cats will not damage the car unless the catalyst starts to break apart and clog/plug the exhaust. Though I'm sure in Cali they wont let you get tabs for the car if it won't pass emissions.

C

[lodit]

Cool.

Thanks again for your assistance.

So, I don't have worn tie-rods, cause steering is fairly tight, no clunking noise, and no alignment issues (car tracks straight hands off the wheel).

Wheels are brand new, as are the tires and rings.

I found a website "macautoparts.com" that sells what look to be OEM cats and O2 sensors for amazingly decent prices...anybody have any experience with them?

Otherwise, I'll just save up for a month or so and buy a complete exhaust system, from downpipes to cats to resonators to mufflers to tips...roughly a $4-5K job for Millteks, wouldn't you say? Unless someone knows where to get comparable parts and competent labor much cheaper.

But it will atleast serve me for another few years...I'm looking to get this baby to 250K miles...before I lay her to rest...a little over half way there now, and still such a beautiful car...

Money pit it may be, but I can't put a price on the smile on my face every day when I stop and stare...I mean, it's an 8 year old car, and I still pause for a moment before I open the door to take it all in...the only other time I do that is when I think of my family...but throwing money at them doesn't cure everything and bring back that lovin' feeling every time...

Lol...if my wife is reading this, I was only kidding, honey...honest...

[Affinitive]

Yeah it is a beautiful car. I have one customer in particular (a couple actually) with an 03 model with the V8. I always pick it up at her office when I need to work on it, so I get to drive it quite a ways to get to my shop and I always have a big huge grin on my face for the entire 10 - 15 mile drive plus they have a really good service contract on it which is a bonus from the Biz perspective.

Anywho, you may wana try www.audiporschevwparts.com. Their a dealer in Cali with an online store and they have really good prices as well and shipping probably would not be much at all considering you're Cali too. Maybe also try calling Sunset Audi/Porsche in Oregon. Just google the name and they will pop at the top of the search. They're a dealer as well with even better prices and you'll be able to avoid the sales tax but probably more shipping though. I just ordered to famous P-Car Water Box Crank Seal tools from them and their prices were by far the best. Those V8's already sound pretty sweet with factory exhaust, so a little aggressive aftermarket cat back or full exhaust would really sound good. Also the V8 models use the more heavy duty 5HP24 transmission which will very easily go 300K if taken care of. Just service it every 60K at the latest and the differentials and transfer case every second trans service as well and you'll be good till 300K, easily. Well,...., couple of timing belt jobs too but you get the drift. That car is not really that much of a money pit, especially not compared to BMTroubleyou's and MB's and for a big V8, it gets pretty decent gas mileage if you think about it.

The vibration, is it only on acceleration??? Or only at certain speeds?? Did it start right after the new tires and wheel swere installed. I don't do tires here myself at my shop, but when I have worked at shops in the past where we did tires, it was fairly comon that we would need to rebalance tires after a couple of months of couple thousand miles. Then of course, there is the customer who calls to complain about a new vibration or more noticeable vibration after rotating a set of tires. I just keep leaning to tires because 95 out of 100 vibration complaints are always due to tires.
C

[lodit]

Thanks once again for your help.

Yep, vibration only when the new wheels/tires were put on.

The company that sold me the wheels is sending out another set of adapter rings, so hopefully that will cure the problem.

If not, then I'll have to get it looked at further.

I'll check out the website you mentioned above and see what they can do.

Btw, I'm in Cleveland, OH right now for work. Won't be back in the OC for a while.

My setup is as follows:

2001 A6 4.2 Q Sport Suspension Black/Black
Koni FSD's
H-Sports Front and Rear Sways
SS Brakelines
5mm H&R spacers all around
MRR HR2 19x9.5 et 35
Falken 452 245/35/19
Yes, the wheel does sit on the hub with the spacers in place, so they are not floating.

I would like to lower it 1" or so all around, but Koni does not recommend it. However, they do authorize using Eibach lowering springs with their Koni FSD's for the 2.7T A6?

Not recognizing the difference there? Other than maybe weight. Are they concerned that the shocks would be under a heavier constant load? More risk of bottoming out?

I called them and they didn't say why, just that it wasn't recommended.

I think with a 1" drop, cleared corners, a new exhaust, spoiler, and a good wash and wax, the car is complete for me. Actually, I don't need the spoiler, want to maintain the executive look.

[Affinitive]

HHHMMM -

I noticed someone else has also made a thread about the hubcentric rings.

Without a doubt, if the aftermarket wheels have a different hub diameter, then it SHOULD have hubcentric rings. Thing is, most of the time I see aftermarket rims which usually have different size hub diameters but no hubcentric rings installed and like mentioned, no problems. Someone in the other htread mentioned the wheel bolts are not designed to hold the weight of the car, well techinically they don't, even on stock rims as there is still a slightl bit of clearance between the hub and wheel. Technically it's the surface tension of the of the wheel being pressed against hub/rotor surface that supports the weight of the car once the wheel bolts are tightened creating that pressure I mentioned.

I was kinda thinking about this a bit last night, (yeah thinking about how I sound like some know-it-all jackass with all the magic answers over some forum,.....LOL) and got to thinking that maybe the wheels you had gotton were designed for conical wheel bolts/lug nuts instead of sphearical wheel bolts/lug nuts. If that's the case, technically you should not be using those wheels with sphearical wheel bolts but I supposse the wheels will form to the sphearical wheel bolts once the bolts get tightened a few times. Probably not all that good of an idea but not probably not the end of the world either, so to say.

Did you have to get different wheel bolts, as in changing to conical bolts instead of using the OE sphearical???
Are you able to remove one of the wheel bolts to see wether there is a straight taper in bolt mounting surface or a curved surface??? If you pull only one bolt, then you don't have to worry about lifting the car off the ground and all that jazz. Ideally wana have a torque wrench as well to tighten again.

Does that make sense what I'm saying about the Conical vs. Sphearical??? Am I even spelling "sphearical" correctly,.....,LOL??????

Think of it kinda like comparing BMW wheel bolts to VW/Audi wheel bolts. If the wheels were in fact designed for conical bolts, I would imagine you should be able find a set and ideally use the hubcentric gadgets as well but save the sphearical bolts in case you ever go back to OE rims,...., if that's the case.

Maybe I'm just ramblin a bunch of nonsense

C

[lodit]

Lol...

Ultimately, it looks like you're asking:

Am I using conical lugs for my aftermarket wheels?

Answer: Yes. Same lug bolts that were on my previous wheels with the same bolt pattern and offset. Torqued to Audi specs...I believe 75/80 lbs-ft...

Now you got me thinking I should check if the wheels are meant for ball seat and not cone seat...lol

Cheers!

[Fleet Admiral]

So what's with those universal fit magnaflow cat listings only saying it's for a 1999 2.8 A6? Don't they make them for 2.7? All I can find are ones that have no support for the O2 sensor(at least the photos don't show-maybe generic pics). And the ones for the 2.7 they only offer the full $400 assembly, and not a $80 cat. What's so hard about precutting bung support for a damn O2 sensor. I realize these are generic, but christ sake. And from most reports, these generic ones are NOT direct fit under the car. They're too big. More cutting involved?

Magnaflow doesn't even list anything on their site for the A6 after 1999. I listed a website ctconvert for stock ones that are around 450, but don't feel like paying it since it's more than it's worth for how long I'll probably keep it. This place from reviews are DIRECT fit, no welding/bending, unlike eastern catalytic ones.

[G Men 08]

damnn someone got the shovel out and started diggin!!

[04A6SLine]

speaking of which.. where are those guys??

Affinitive seems to know a lot....

[xpl0sive]

who needs cats anyways.. cut them out and weld in straight pipe... put a spacer in for the O2 sensors and no CEL... that was my solution to a bad cat... and then i took my cats to a scrap yard and got $150 for them... so that paid for the job to cut them out and weld in straight pipes

[Fleet Admiral]

And what effect does that have on engine management and fuel economy? I can lazily get 30 mpg by easing the pedal.

[xpl0sive]

well i've got a 4.2, so my MPG was never great anyway... i'm amost never on the highway, mostly city commuting, so i'd be lucky to get 18 MPG before or after getting the cuts removed. no effect on mileage without cats...

[Stone825]

http://www.randomtechnology.com/products.html

You could always do that (unless you live in California).

[Fleet Admiral]

None on that site.

So, is it better to just order an $90 ECS univ cat, or go the whole route and get the assembly for $400? The codes are warm up efficiency, and can hear some rattling when cold. The ceramic probably broke loose. I'm just on the fence with regard to only replacing the big cat itself or if it's related to the first O2 sensor and not the 2nd in front of the big cat.

Another thing is the ECS has no O2 bung, but I read Magnaflow does, but I haven't been able to find any except for the 2.8

[soccerplyr]

I have the same issue with my passenger side cats. They are pinging something awful. Sounds like I'm dragging tin cans under my car.....

I found a direct fit on ebay from exhaust distribution. $300 plus shipping.

Details:

FEATURES AND BENEFITS

Industry's most comprehensive coverage.
409 stainless steel shell and neck construction for longer life.
OBDII coverage.
Manufactured from high quality materials.
Design meets EPA emission requirements as cataloged.
100% aluminized pipes are constructed from heavy 14 and 16-gauge steel.
Re-aluminized weld seam to prevent corrosion.
Precision engineered fit for easy installation.

Direct-Fit catalytic converters are designed for easy bolt-on installation for the professional installer. Welding, cutting, and tube-bending equipment are not required. Each is designed to exact original equipment specifications.

They feature a ribbed body that minimizes expansion and distortion when the converters heat up.The ribs form a channel that protects the cushioning mat from direct exposure to exhaust gases, and they hold the ceramic catalyst in proper alignment.The ceramic coating is designed for maximum flow and surface area, and the coating method is strictly controlled to maintain exact specifications.

My CEL is on as well and has been for 6 months. What else do I need to buy?

Thanks!

[A62TURBO]

^^^ Not bad, but remember that you get what you pay for.

[soccerplyr]

Educate me, please. Do you see anything wrong with this product?

Thanks.

[HOFFY11]

NO CATS, no resinator, mufflers..and 2.5inch piping

[Fleet Admiral]

NO CATS, no resinator, mufflers..and 2.5inch piping

Like I mentioned before, doesn't that throw a code? How does it affect mileage/fuel mixture due to the fumes not being dealt with properly. Not to mention it's illegal and difficult to find someone willing to do it.

I found a direct fit on ebay from exhaust distribution. $300 plus shipping

I wouldn't trust them. I've read everything I could find from multiple forums and decided it's either ctconvert.com with their TRUE no hassle direct fit(unlike eastern cat ones) for abt $450 or just weld in a magnaflow, but I haven't found any with O2 bungs.

designed for easy bolt-on installation for the professional installer

More like the professional wrestler.

[mattsimis]

I have no pre or main Cats in the Allroad. No CEL or DTCs at all using the "Wayne Angled O2 Block" on the rear sensors. Have to lol at "Audis have extremely sophisticated O2 monitoring" statement at the start of this thread. Apparently not as sophisticated as a square lump of metal..
Regarding "fumes" not being dealt with properly.. thats not what an Cat even does. MPG goes up as you have less back pressure and lower heat. Better for Turbos too. Very little noise difference too.

Im converting the Car to E85, on its 4th full tank. Only smell I (after the de-cat) is a faint sweet apple smell. Should pass emissions on straight E85 no cats too, already checked with one of the test center guys.

[soccerplyr]

You sure it doesn't have CATS? What do I see in the picture?

http://cgi.ebay.com/ebaymotors/ws/eB...m=350193281395

[Fleet Admiral]

You sure it doesn't have CATS? What do I see in the picture?

He means he removed his. Modified exhaust. I'm not ready to do that. I just want to know what's best, cut/weld a magnaflow in there if I can find one for the 2.7 and not a 2.8 without going entire custom downpipe, or get a OEM aftermarket driver side because it has cat/precat.

It's throwing code saying inefficient catalyst warmup, which can mean a lot of things in the fuel system, but I know it's the cat because it rattles when cold SOMETIMES, which means something is loose and I don't want it to get clogged and blow a turbo or whatever else could happen.

[chrisbostonusmc]

fvck cats and the environment...let the ricers smell fuel when they're behind me

[wofman]

Let me try to simplify: cats are used to scrub emissions of toxic gasses -- oxidize hydrocarbons, nitrogen, etc. O2 sensors measure success in doing so. (If relocated they become inaaccurate -- a false positive -- often used with cats removed.)

Stock cats restrict flow, cause loss of power. Removing them increases power significantly. Free flow cats (race cats, performance cats) offer best of both worlds -- green and mean.

[doobiesdaddy]

My cats went bad last year. I got the APR downpipes and high flow cats, plus the cat back exhaust. It was expensive, but worth every penny(in my opinion). It was probably the best mod I have done to this car. No CEL and passed CA emissions. Now my turbos spool quicker and it sounds awesome