DeCat Yet Again!


Registered User
Nov 9, 2008
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Darlington, England
Everytime I post about this I never get an answer about it. Please could anyone tell me. Do you have a decat pipe?

If so/or if they know of does the antifouler modification keep the ECU light off or does it come back on ever so often?

I want to buy one but havent due to the uncertaintly over the ECU light been stuck on and due to the hassle to install it. I only want to have to do it once.

PLEASE can someone draw a line under this for me. Im fed up of trying to get an answer.

Many Thanks
Can't do definative answers but I have been looking into this too.

The ECU on the A4 has sensors before and after the CAT. The car uses these sensors to monitor the effectiveness of the CAT. The sensor after the CAT monitors exhaust gases to see if the CAT has done its job. This is part of the Euro 4 regulations. If you remove the CAT the sensor sill see the exhaust gases have not been treated and are out of range. The anti-fouler mod shows the sensor values that should be seen by the ECU as if the CAT was there. This has mixed reviews. The ECU compares the before and after gas analysis and checks it to a set of given values. If either vlaue is out of the set range the car shows an error message and requests service.

Some people say the anti-fouler mod is a fit and forget, some people have trouble with regular error messages. This seems to be down to luck. Re-maps which alter the emissions expectations can cause more error mesaages to pop up.

I know this isn't much help, but in reality a de-CAT pipe will yield similar results to a sport CAT. If you are worried error messages then go down that route instead.
AndyMac Thanks for the reply. As you can see from the views to response ratio this is a taboo subject. In my opinion most people who want more power from an engine need to decat as its USUALLY one of the most easiest things to do and release good power gains. There should be some sort of sticky that has some definative comments about decats to put this subject to rest. You have confirmed what I already thought.

Its alittle hit or miss. Trouble is a sports cat is ££££££ and a decat £65-80. So the decat seems the way I want to go but dont want to fit it to find out the ECU light is on and off light a light bulb.

I wish someone who has experience with these would reply and either put my mind at rest or definitively say it will not work.

Many Thanks Andy
There's nothing taboo about it.

It's just a pain in the ar** having to swap the test pipe/Cat' over at every MOT for what is going to be not much of a gain. It will bring the emissions light on because (as stated) the A4 has a closed loop system using upstream and downstream lambda sensors.

Have you got your car remapped? In the 'States some of the software companies can stop the light coming in but I would say it might prove difficult to get it done over here. Plenty of info' on AudiWorld, AudiZine and AudiGeeks.

Apart from that what's stopping you?

Thanks for the reply Bubstar.

Yes the car is remapped upto 190bhp (From 150) I have no mods yet except a exhaust manifold im putting on today. Im no expert but on other cars ive had removing the cat usually yields good gains. Thats why I wanted to do it. I havent done it yet for the exact reason you stated. Its a pain in the a**e to put it on to have to take it straight back off if the ECU light comes on.

But what about the antifouler mod? Does this work? If not then there is my answer. I dont see the point of going through all the trouble of putting it on and have to get a software company to stop the light coming on. Thats an expensive way to get a small gain (In terms of BHP to £ ratio).

I read on the yank forums that the decat (or testpipe as they call it) only gives a couple of hp over the standard cat, after all to get a max of 225bhp from a 1.8 fourpot is a bit of a magical feat anyway so most of the oem parts must be pretty close to perfect than other cars.

I considered it too mate but I think its too much grief for 3 to 5 bhp if thats all it does.
hi all
it is possible to do the decat but you need to remove the second lamba sensor the only way i've seen it done , was some one made up a little metal shroud to cover the sensors end and is attached to the bottom of the car and the hole in the exhaust is blocked up and this keeps the ecu light off .
i hope this helps
the decat is worth it , it's not all about the power it also makes the throttle a lot more resposive as well .

Thank you for that. Please could you expand on what you said. Or can someone else please clarify what has been said?

As previously said I would like to decat if I knew the ECU light would definatly stay off.

hi there ronniebiggs.
its like this there are sensors before and after the cat so u have to remove the second sensor from the exhaust , keep it connected to the car but u have to cover the end over or the water from the road will destroy it this will stop the light from coming on . the biggest problem your going to find is someone that makes a decat pipe .
Thanks Worspeed

THS make them but they rip you off. They started with a price of £70 plus postage went down to £60 as they never sold them. Then when I asked them if they had any for sale they listed one straight away at £90 plus postage:no:. So I e-mailed them and said I had seen them at £60 from them and they said the price of materials had gone up:think:. If I had a pound for every time someone says that!

They obviously saw an opportunity to charge more as they had none listed and tried there luck!

Apart from them I havent seen any decat pipes. I was going to buy one from them but not unless they drop the price back to £60.

Does anyone else know of any other decat suppliers?
I have removed both of my pre-cats and passed the MOT. Currently I do get error messages but the main cat sensors will be relocated ot the rear and tis should solve the problem. The reason for the messages is due to all 4 sensros getting the same reading. Fuel consumption on a long run is better although around town it is pants. Moreso than normal. Got no bhp/lbft figures but will do some performance logs soon. I bought new bosses and blanking plugs of fleabay for approx. £20 incl. P+P.

The car is a S4.
Let me know how it goes. Is your car set up like said below?

Eg have you still got the sensor near the engine side in and the after one with a cap over the top and attached to the car? Or have you simply removed the sensors all together?

My car has a straight downpipe from the exhaust manifold with the sensors installed. The main cats are 100 cell but the sensors are in front of the cats. Therefore, al four sensors are reading the same value resulting in erros andan eventual CEL. I am gonig to move the main cat sensors to behind the 100 cell cats. This will give different sensors reading and solve the error/CEL.
What car is this on?

What does that mean explained simply? You are moving the sensors. But are you having them in the exhaust and hence they are in the gas flow. Or are you putting them anywhere under the car so its not in the exhaust gas flow?
The car is a B6 S4. I will be welding new bosses AFTER the main cat(s) and plugging the old ones with blanking plugs. The sensor(s) will then be screwed in and the probe(s) WILL be in the gaseous flow. Have a search on Audizine for "piggies" or search the VASTPERFORMANCE website for pictures.
Everyone says the antifouler Mod is the way with the 1.8T. However I havent really had confirmation that it keeps the ECU light off. The only thing that appears to be a concrete working way is described below where you remove the after sensor and block the hole. Then make a sleive for the end of the after sensor and leaving it attached to the car strap some how to the car chassis.

But only 1 person has said this method works so again its alittle scetchy going for it on only 1 persons advice. The only way forward is to do it and take a gamble. Then report the findings. However I havent found one at the moment thats a reasonable price to be worth taking the gamble.:nyah:
Have you checked over on Audizine mate??The yanks love this kinda thing..

Thepre-cat sensor and post cat sensor (that you have only the 2 of) must show different readings, before cat treatment of the exhaust gas and after, so the car knows the cat is working. If you fit a decat pipe, both sensors would now read the same and trigger a CEL light and a fault code.

The best mod is to get the after-cat sensor out of the gas flow and into a sleeve but probably best a dead-end threaded boss welded to the pipe after the decat pipe that the sensor can screw into, thus protecting it, but dead-ended so it still shows a clean result. Then block or insert a bolt or otherwise blank the original hole for it so theres no leaks, job done.

The precat sensor should stay where it is.
@Ronnie: Just relocate the sensor it as easy as anti fouler or sleeves etc. By doing what ESM recommends makes the 2nd sensor redundant from its intended job. That is to determine if your main cats is functioning correctly. This could cause you problems if the cat has failed and you go for an MOT without knowing.

@Juss. It is the same principle for 1.8T in that there are sensors at the precat and main cat. Obviously the V8 has 2 more but the sensors are positioned in the same places.
Thanks guys, this is an interest ill be looking into. I might even have a source that can make the Pipe for me - just depends on how much it will cost me!
Give me a shout if you do get one made!:yahoo:I will have one off you if the price is right. Let me know J7USS if you get one and what happens. Thx for the advice guys.

One other question though. If the post sensor is in a sleave that presumably will be air tight. Will that not give a false reading or ECU light?

Does the sensor not need to be still exposed to the air itself. If it does then water will be able to access the sensor.

Have searched and asked on other forums. Not Audizine as im not registered yet. Had no luck. This is by far the best response ive had on the subject in the 2 or 3 times I posted.

Im almost at the point now where I will try it. Just need to iron out these minor things before I go ahead.

One other question though. If the post sensor is in a sleave that presumably will be air tight. Will that not give a false reading or ECU light?

Ronnie it has been pointed out on numerous of the above posts that the whole entire point of putting the sensor in a sleeve or plug anti-fouler is to give a false reading to the ECU in order that there are no errors or CEL. The pre and post sensors are supposed to give different readings. Removing the pre-cat results in both sensors giving the same reading = error/CEL.

Does the sensor not need to be still exposed to the air itself. If it does then water will be able to access the sensor.
Water is not an issue. See my post and others above.

Join Audizine and get the job done. It is not a big issue.
just screw a bolt into the original sensor hole so you can put the sensor back at mot time, be aware though that these sensors will likely be a weird thread size, like BSW or something, I could not match one to any nut in the engineering stores at work when I was repairing a downpipe for a mate. It was neither metric nor A/F, but something else instead, even gas fittings and water fittings were different, I ended up TiG welding the sensor into a hole drilled in the new pipe.
Evil - Have you got the facilities to make these pipes by any chance??
You'd make a Killing...
I did at my last job j7uss mate, they fabricated machinery parts on the fly for industrial maintenance in 24/7 environments and had all the toys.

Unfortunately now im in a defence electronics plant and not in the maintenance bit where they have some fab equipment, for my subaru I took the cat off a scrapper, cut the flanges off leaving a pipe stub for angle, and fitted a stainless pipe between the flanges and welded it shut, you can weld stainless to plain steel using a TiG welder and stainless rods.

I would have thought these would be easier to get because jap motors use decat pipes like water and they are quite cheap! German hardware on the other hand is euro friendly eh lol.

One "cheap" way I heard of, is to get a used cat and punch the honeycomb innards out of it using a wooden dowel like a broom handle and turn it into a decat pipe...

I'll se what I can do mate, if I can find someone round here wi a workshop I can use I might buy a TiG welder of my own cos I could do lots of cool stuff with that, good idea!
Yeah Evil that sounds good. You wouldnt have any probs selling them without a doubt especially if you are coming in cheap but obviously making a neat profit - Imagine the Yanks!!

The Guy who made his own Pressure tester on (think that was the forum anyway:think:), he made few more for forum members then slaped a load on Ebay and made a mint...still sells a few now!
Hong Kong are selling the Decat pipes for £130 odd but seems pricey to me.
You know all the ins n outs, know what your talking about and have the knowledge by sounds of it so time to Cash In mate :yes:, Why not eh!!!!
not a bad idea mate if I built them to order, using standard dimensions for the oem cat entry and exit pipes. I don't know how many folk would buy one though, but you never know.

Once you have the setup right a jig can be made so subsequent ones are easier to build with the angles right etc. I may try buying old scrap cats to see what I can do with them....
Go for it Evil. Like i said the overseas will be good if you can source a reasonable shipping rate. Once you explain to the folk out there how easy it is to Decat the A4 then I reckon you'll have them flying out. Even if its a dozen of them - theres stil the opportunity to make yourself a Few quid!!
They must have dropped them again because they couldnt sell them. They just put what ever price they feel at the time!

Still undecided on them J7USS. Would like the performance change from it but Im getting alittle worried about the insurance side of things now. If you put the decat on then in theory you invalidate your insurance for a few reasons.

Obviously because you added a mod without telling them.
And also because the emissions wouldnt be legal. Now you would never get caught out for that unless the ministry pulled you up and checked but I have never heard of that happening.

I used to have a decat on my Impreza.

Not sure what to do is the honest answer. Last time I played about down there I fitted that stainless manifold and it was a nightmare. Im sure the manifold is leaking exhaust gases slightly! and I physically couldnt get 2 of the buts on the manifold:scared2:. So to possible have to take that all apart again is a little scary:tapedshut:

E-mailed the seller about it as he lives not very far away from me so if I bought it. I could collect it from him.
Just reallised I replied thinking it was THS that was selling it. So ignore my rant on the last reply about THS making up prices. Howver it is still true!:)

Yeah I decat'd my Scooby too and WOW it made such difference BUT on our A4's I dont think you would see mega difference really plus it'll prob sound Raspy aswell.
Im toying with Miltek Sports cat + full Exhaust, Just so pricey though...
I like the idea of the sports cat but couldnt justify the huge cost for the small if any gain.

Thats why I was considering the cheap method:). Still undecided though. I would agree the tone would change maybe for the worst. It changed when I added the stainless manifold and again im not sure if I like it or not.
yeah I know where you coming from mate, Its a **** load of money to spend on some pipe work!! Ill find that link for the N75 aswell later when home from work. I got it somewhere, sure I have...:think: