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[Audi A3/S3] Decat - legal to remove pre '91?


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I have recently read that it is currently legal to remove a cat and replace with a decat pipe to any catalysed car first registered before 1 August 1992.

Can anyone clarify this?

Also, does anyone know what lambda sensors the A3 TDi Sport(51 reg) has - are they before and after cat or just before?


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Yes I believe this is correct. I used to run my 95 Mini Cooper 1.3i with a decat pipe and just used to put it back on for the MOT. Obviously I was running the risk of getting done by the Police in road side checks etc but the chances are slim. And with a 1991 registered car you will be fine.

BTW the performance difference it made was surprisingly significant but that was on a relatively straight forward engine with very little in the way of electronics and sensors. Should be OK on your 205 I would think.

Does anyone know what would happen if you de-cat an S3, does it go like stink or run like a bag of spanners?

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I always thought it fell upto about J reg, so that sounds about right.

I wouldnt waste your time decatting an S3.

A. Illegal. £1000 fine iirc if caught.Unlikely, but you need to know the penalty. I will admit however, ive only ever seen 1 roadside check in 10 years driving.

B. A few have done it, and i dont know of anyone thats kept it. (their reasons unknown)

C. Dependant on year, early S3's have one lamda before the cat. Later ones have before and after cat lamda sensors.

the former easier to decat without ecu faults.

the latter, less so. The ecu expects to see an improvement in the emmisions, comparing the two readings. If it doesnt, it will flag a fault and shut the car down into limp home mode.

D. Sports cats do largely sod all on the S3, so the cats are not the restriction. Its well known the S3 exhaust is good. Its the ports on the mainfold thats the restriction.

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Cheers for that Paul,

I did once get stopped for a roadside emissions check but it was about 7 years ago and I suspect I was only targeted as I had an old car (1989 Vauxhall Nova 1.0). Luckily my transport has improved a bit in that time! Anyway the car wasn’t fully up to temperature so they just let me go.

Plus from what I’ve read the ONLY sensible way of improving performance is to get it re-mapped. I was just intrested to see if de-cating helped.

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