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DECAT Passing MOT Emissions Test


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Hi, this forums been a good source of helpful advise and information since owning my Impreza, so today thought i'd share some of my recent findings regarding de-cat'ing certain sections of the exhaust system and if a newage WRX car would still pass a standard UK MOT emissions test.

My Impreza's an 05 WRX Blob, the engines basically stock, and with the only mod being a Hayward and Scott 2.5''  resonated rear centre section and back box ( retaining all 3 standard catalytic converters)  it made 269BHP when re-mapped at TDR in Warwick. Shortly after that it had its MOT and passed its emissions test as expected with ease.

After a while (and rather typically) i've become used to the power and in the new year will be looking to make a bit more, so it now has cooler plugs, HKS panel filter, walbro 255 fuel pump and most importantly a gutted/decatted stock up-pipe and a Japspeed decat down pipe, whilst retaining the standard centre CAT. (I also have 650cc injectors to fit but wont be doing this until the car is re-mapped again)

I popped into see my local MOT tester the other day as i was curious to see if the car would pass a UK MOT emissions test, or even be affected by having 2 out of 3 CATS  removed.

As can be seen from the photo below showing both results the emissions test shows to be basically un-affected by the removals, and passes with flying colours by just using the centre CAT. Thought this information might be of help to anyone looking to de-cat some of their exhaust, but curious if they'd still pass an MOT. Thanks for looking, TomIMG_5342.thumb.JPG.faf5148d8f1e28af33e3e1f03ee9a9a9.JPG

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  • 2 weeks later...
On 22/12/2017 at 8:37 PM, Scoobyghost said:

Smells a bit fishy. With no catalytic converter there is no way a car will pass an emission test.

Were the garage aware of it being a decat?

Is it actually decat or are you using the term decat meaning non resonated?

The OP never said they'd totally decatted, though. They have removed 2 out of 3 cats. Obviously the remaining cat is perfectly adequate in terms of the MOT test. Technically running with no cat at all is an automatic MOT failure irrespective of engine emmissions.

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The OP never said they'd totally decatted, though. They have removed 2 out of 3 cats. Obviously the remaining cat is perfectly adequate in terms of the MOT test. Technically running with no cat at all is an automatic MOT failure irrespective of engine emmissions.
A blob only has 2 catalytic converters commonly referred to as "cats". The 3rd box is simply a resonating box used to quieten down an exhaust. A lot of the time a resonator will be incorrectly identified as a "cat". Leaving that box on will not affect an emissions result. The op reads as having removed the up pipe and down pipe cat, leaving the car completely decat.

Obviously a car with absolutely no "cats" will always fail an emissions test.
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