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Registered 2002 SLK320/2001 SLK320
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66 Posts
I am in the process of getting a Powerflow exhaust system in the UK.

The new exhaust system is going to go from the 2 inner catalytic converters near the manifold, removing the additional 2 cats further down the exhaust. So I will be left with 2 cats instead of 4.

Has anyone done this and if so, were the MOT fail issues going forward?
 

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Registered 2005 SLK350
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121 Posts
I believe the biggest problem you will face is not one of emissions, but whether the tester will allow a pass when two of the four standard cats are not physically present. I am no expert but I think the 'normal' MoT requires all standard cats to be fitted and working.
 

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Premium Member 2014 SLK55 AMG
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1,658 Posts
De-catted ( 2nd cats ) on my SLK 55 AMG and it made zero difference to the emissions.

The MOT operator wouldn't have known how many cats were fitted to a particular car anyway!

Reinstated the system before selling and the new buyer has just put some straight throughs on the car. Sounds brutal!
 

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Premium Member 2012 SLK55 AMG
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2,502 Posts
Keeping the primary cats you should have no problem passing the emissions test! The secondary cats don't do a lot of work. They are there to catch anything the primaries miss whilst they are still coming up to temperature. Plenty of guys on here from the UK have done this on the 55 AMG model without issue. Some even claim to have passed an emissions test on just secondary cats after removing the primaries when fitting long tube headers. MOT testing will always be done with pre-warmed cats. Most MOT testers are ignorant to how many cats cars are supposed to have. I can't see you having an issue unless there's something in the new rules coming out this year?
 

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Premium Member 2005 SLK55 AMG
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11,404 Posts
As has been said, no problem with those of us that have had the secondaries removed (and x-pipe to replace the centre resonator). Not exactly what you have planned, but check out the emissions link in my signature for some before and after numbers...


However, if you're planning a complete de-cat at some point, then that may be an entirely different story. ;)
 

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Registered 2005 SLK350
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121 Posts
I'm not trying to start an argument or be Mr. Clever-dick, but just to back up my previous statement have a look at the MoT test manual from the .gov.uk site, section 7.1.3
https://www.mot-testing.service.gov.uk/documents/manuals/m4s07000101.htm

Whether or not any tester would know how many cats should be in place, the regs allow for failure if they are missing. That was my sole point.
 

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Premium Member 2014 SLK200
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2,168 Posts
I'm not trying to start an argument or be Mr. Clever-dick, but just to back up my previous statement have a look at the MoT test manual from the .gov.uk site, section 7.1.3
https://www.mot-testing.service.gov.uk/documents/manuals/m4s07000101.htm

Whether or not any tester would know how many cats should be in place, the regs allow for failure if they are missing. That was my sole point.
I imagine that, should an inspector's suspicions be roused by obvious changes/adaptions to an exhaust system, they MAY look into things further.

Certainly the new regulations and guidance for testers will be far more strict with regard to removal of DPFs from diesel vehicles.

Would be worth speaking to your local authority MOT station to see what their position is.
 

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Premium Member 2014 SLK55 AMG
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1,658 Posts
Fit dummy or gutted 2nd cats
 

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Super Moderator UK SLK 55 AMG 2007
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27,273 Posts
As has been said, no problem with those of us that have had the secondaries removed (and x-pipe to replace the centre resonator). Not exactly what you have planned, but check out the emissions link in my signature for some before and after numbers...


However, if you're planning a complete de-cat at some point, then that may be an entirely different story. ;)
I'm not trying to start an argument or be Mr. Clever-dick, but just to back up my previous statement have a look at the MoT test manual from the .gov.uk site, section 7.1.3
https://www.mot-testing.service.gov.uk/documents/manuals/m4s07000101.htm

Whether or not any tester would know how many cats should be in place, the regs allow for failure if they are missing. That was my sole point.
Mot testing has little profit unless fail work is done at the garage.
Most fails that come up seem to be the quick to fix unless rust/structure is involved.

If dummy boxes fitted, they'd likely get through if emissions test is ok.

But, as stated, if they spot it they can fail.
However, if you have bolt on dummies, you can be quickly road legal on a retest.

Unless they have/will add a rule about complete systems and not bolts.

Often discussed at meets.
I don't recall anyone getting failed. So if there are fails they are few.
 

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Super Moderator UK SLK 55 AMG 2007
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27,273 Posts
Yup, kind of underlines the fact that some emissions equipment fitted by manufacturers was unnecessary in the first place.
Who has been conning who?
I suspect they looked at Ca, USA (and others).

Build one car for as many markets as possible.

Buyer will pay.
 
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