Not sure I would agree with that statement.
Yes, discussed to death before but…
Copy from my previous post.
For your information if you are thinking of modifying or indeed already have modified your exhaust system. In particular the NC.
This topic of people saying it’s an instant MOT failure if you remove the front Catalytic converter is just not correct. Or indeed removing the centre one and keeping the front one.
It was and always has been my understanding that as long as the car meets current and the designed emissions standards for any car then you can delete either the front or centre Catalytic converters as in the MX5.
Likewise, you can modify the exhaust system as long as it complies with the noise (perhaps another topic)and the emissions.
People state BBR are correct is stating it will fail if you remove one.
That is not correct and in my opinion is a sales ploy, which is fine by me.
I have therefore contacted the DVSA for an answer.
Please see my enquiry and the reply I have received.
All a matter for your good selves, but some people will never accept this information of course.
Hi,
I am wondering whether you can give me some help and advice on Catalytic Converters and Emissions for Spark Ignition Engines.
I have read sections in 8.2.1 as attached below, but to me and many others is a bit of a grey area.
Some cars have two catalytic converters and indeed some only one.
Those that have two fitted as an example may have one built into the manifold/header pipe and the other will be inline somewhere.
Some just have the two fitted inline within the system.
What I would like to know is what happens when someone wishes to fit a complete road legal stainless steel exhaust system.
EXAMPLE.
Removal of the front header and catalytic converter and be replaced by a standard after market 4-1 stainless steel manifold.
Replace the centre OEM catalytic converter and exhaust section (or keep just the catalytic converter) and replace with a new one and stainless exhaust sections to tail pipe including silencers etc…
In doing so this would fully meet or exceed the current emissions and therefore pass the test.
Now as you also know there are many “after market type approved items” on sale that can be fitted legally to vehicles and still pass the current emissions tests for cars.
Indeed they are recommended as direct replacements.
Quite a few companies can supply or indeed build them to specification.
Having read the below, to me it is slightly confusing because of the part about “Emission control equipment fitted by the manufacturer, missing, modified or obviously defective”.
So can you replace/modify the exhaust/emission control systems with aftermarket components so that it still controls and complies with the current emissions standards?
Am I correct in saying that this was written to stop people just deleting the emission control systems altogether?
Also, if say a catalytic converter should fail then a person could replace with an aftermarket one which fits into the category of not being fitted as standard and obviously modified.
BUT is type approved and legal as far as the emissions test goes.
The same logic can be applied to the complete exhaust system.
Likewise it would be impossible for a Tester to know or understand the complete exhaust/emissions systems on all road cars.
So am I correct in saying the tester would inspect for a Catalytic converter and conduct an emissions test in the normal way?
I look forward to hearing from you.
Kind regards,
THE REPLY ON THE 3RD FEBRUARY 2021.
Exhausts can be a problematic area in the MOT test. This is largely because the test is based upon the relevant legislation and this sometimes makes it difficult to assess compliance where modifications have been made or aftermarket components are fitted.
Firstly, it is illegal to replace or modify an exhaust so that it increases the noise made by the escape of exhaust gases (Road Vehicles Construction and Use Regulations 1986 -Regulation 54)
It is also illegal to make modifications to a vehicle that increases the levels of exhaust emissions (Road Vehicles Construction and Use Regulations 1986 -Regulation 61A). This is not just the emissions checked at MOT test, but all emissions that that must be met as part of the type approval process.
Therefore, if an aftermarket stainless steel exhaust is fitted, the MOT tester will need to assess that the exhaust noise is not unreasonably above the noise level you’d expect from a similar vehicle with a standard silencer in average condition. They would also check that no emissions control components are missing, obviously modified or obviously defective.
*It is important to note that the regulations do not prevent modifications or the use of aftermarket components, including exhausts. The components are merely required to perform as well the OE components in respect of noise and emissions. If the design of the exhaust uses one catalyst where there were originally two, this is not necessarily an MOT failure, provided the system still meets the same standard for noise and emissions as the OE components did when in a serviceable condition. It may be necessary to show a receipt or other documentation to prove this, but like you say, most testers will not know how many catalysts were originally fitted.
In all cases, where there is doubt about whether a defect exists, testers should give the benefit of the doubt and pass the vehicle.
I hope this helps.
Best regards
Shaun Martin | Policy Specialist (MOT Testing Service)
Driver and Vehicle Standards Agency | Berkeley House, Croydon Street, Bristol BS5 0DA
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Remember you do get what you pay for. If you do well done and good luck. 
I do know I’d much rather get a 4-1 and center section with a remap than a full exhaust without remap. Backboxes are expensive on the NC because of the size and dual exit, while they are probably the least restrictive part of the stock exhaust and I believe they’re even from S/S. I would also much rather spend the money on suspension or brakes improvements than on the backbox.