All Things Mk5 > Performance Modifications

Can fit a standard cat back on to a decatted car?

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owheel:
Hi guys, bit of a weird question.

Just under a year ago I bought a mk5 gti that turned out to be decatted. I took it back to the trader who changed the exhaust and said he sorted it out and showed me a new emissions report. I thought this was all good and forgot about it.

Unfortunately I had a pre-MOT checkup which flagged that it would fail an emissions due to the cat being chopped out and replaced with a straight pipe.

The fact that the car has been decatted would suggest that it has been remapped. If I fit a standard cat back onto the car, is this likely to cause any issues? My main goal is just to pass the MOT, I don't mind if I'm down a bit on power

Thanks in advance!

pudding:
So it's a stock downpipe with the second cat chopped out, and attached to that is an aftermarket exhaust?

You can attach a factory catback to pretty much anything with the right adaptor, but it won't help with the emissions if the cats are missing.

Octoparrot:
Yes you can, it may affect performance slightly but shouldn't cause any other problems,.

owheel:

I've realised I wasn't very clear haha!


--- Quote from: owheel on April 05, 2021, 03:42:57 pm ---If I fit a standard cat back onto the car

--- End quote ---
^ I meant re install a standard downpipe and catalytic converter onto the car

The current state is that the car has a standard exhaust from the turbo back. The precat straight after the turbo on the downpipe has had the actual catayltic filter removed. The main cat has also been removed and a straight pipe has been welded in place. This effectively has decatted the car (in the cheapest way possible).

It's promising to hear that the car will not suffer from having a cat re-added though! I was wondering whether adding back pressure in the exhaust without having the map changed would cause a problem.

I'm tossing up between fitting a stock downpipe and cats, or welding in a 200 cell magnaflow sports cat


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Jons1001:
I had a the same problem. My car has been mapped and had a decat downpipe when I bought it so wouldn't pass an mot. I had a Magnaflow sports cat welded in which got it through the mot. About 3 weeks later I got an engine management light and following codes;



There were not other symptoms,car appears to be running fine and mpg is as expected. I cleared the codes but they have returned 3 times now after about 3 weeks each time. I contacted the company that remapped it and they said the codes would be because the car was mapped as having no restrictions and I have placed a restriction there (the cat) so would need to have the map tweaked at a cost of £150. I haven't had it done yet but Im going to book it in very soon as it's just bothering me that it might be doing damage somewhere. Also the thermostat seems to have gone so I'm not sure if this could be causing any of the codes, does seem much more likely to be the cat/map situation though.

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