Exhausts, Catalytic Converters and Emissions

Added 5/2/21).
In addition, various models over the years have different EURO EMISSION RATINGS. Therefore, each rating had different emission values to the previous one. So it is important to check which rating you have. Clearly the ND’s will be the latest with the most current stringent values applied.
Just my opinion that if I had the latest spec ND I probably wouldn’t bother swapping modifying as it’s not really worth the bother. I know not whether they will pass a current emissions test with a decat manifold.
Just an opinion, but if you do I would swap with like for like, i.e. get the catted manifold.

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.

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 a someone wishes to fit a complete road legal stainless steel exhaust system.


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,


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


Thank you for taking the time to help clarify this thorny topic. :+1:

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Your welcome. :+1:
I was quite careful not to be model specific too.
At the end of the day it clarified it for me.
Pretty much black and white in print.
But some still think it’s interpretation. :man_shrugging:
It’s important to read the whole reply as one and not pick bits out of it to suit. :+1::slightly_smiling_face:

Would I be right in saying that fitting a stainless steel back box from mx5parts to my MK2 car is legal and would pass an MOT then?

If it complies with the above reference noise level then I would say yes.

It’s far from B&W, lord knows how you can’t see it.
The book clearly states:
“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)”

So straight away you’re in breach of that rule. If a tester wanted to he could uphold it and fail your car.


“provided the system still meets the same standard for noise and emissions as the OE components did when in a serviceable condition” and “This is not just the emissions checked at MOT test, but all emissions that must be met as part of the type approval process”

Aftermarket components such as from BBR do not have type approval and do not meet OE standards.
They do however pass our emissions test. It just so happens that our test is not as strict as the one that Mazda had to comply with.

So those are the actual rules. He then goes onto say “In all cases, where there is doubt about whether a defect exists, testers should give the benefit of the doubt and pass the vehicle”

Which as said multiple times, it’s down to the discretion of the tester whether to pass or fail.

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As far as I can tell it does comply

I don’t intend to get into an online testosterone war with you and matters should be kept light and friendly.
Yes, I can read and I have read the sections fully.
DVSA have expanded on that and replied and advised.
Obviously, other people have read what I have sent and have received and therefore have formed their own opinion.
You are adamant what you say is correct and that is fine by me.
As I said that’s your interpretation of the statement.
Thanks for taking the time to post your opinion. :slightly_smiling_face:

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I recently read Adrian Neweys book on his F1 career. As you may know he’s a famous designer and is employed to design a car which is as quick and reliable as possible by interpreting the rule book. It’s a good read, I recommend it.

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Finally from me to you on this subject.
(As I have read other posts/replies of yours and actually like and agree with some them as you are aware). :+1:

Thank you for the advice, yes I know of the famous Adrian Newey.
But, he does not apply his work to the Road Traffic Act regulations concerning the use of motor vehicles on a road.

Likewise, I have read many Road Traffic and Case Laws including the Construction and Use Regulations on hundreds of matters where you have to apply the whole law/regulation impartially.

All the best. :slightly_smiling_face:

The rules are black and white: it’s illegal to fit a louder exhaust. The reality is a lot greyer: the MOT tester can’t measure noise and has to use their subjective judgement.

Since they have to give the benefit of the doubt, your loud aftermarket exhaust would have to be very obviously louder than a standard one to fail an MOT.

Another possible problem would be if the police stopped you for some reason and you rubbed them up the wrong way. An obnoxiously loud exhaust would be just the sort of thing they could pick on to spoil your day.

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Many thanks.
My MAIN clarification enquiry was reference the Emissions and Catalytic Converter/s.

Well, there there’s another source of greyness in the black and white. The regs say it’s illegal to increase the car’s emissions and removing part of the original emissions control would logically do exactly that, so taking one of two cats off the car would be against the letter of the regulation. But that’s Construction and Use Regs, not the MOT. The MOT tester is instructed that it’s a major fail to have “Emission control equipment fitted by the manufacturer: missing, obviously modified or obviously defective”. Plus of course it still has to pass emissions levels for the unmodified car.

It’s a bit vague because the MOT tester isn’t expected to memorise what the OEM parts look like on every model of every car, So they wouldn’t necessarily spot that something is missing but it’s a fail if they do, except that the DVSA make clear in their letter to you that they intend that to mean a replacement with a non-OEM system is okay, so long as the car still passes emissions.

So that leaves a bit of a gap where e.g. taking off one of the two cats is contrary to the regs, but not an MOT fail unless it no longer meets emissions.

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Remember that the Tester will look up the Emission range parameters specifically for each model of car and apply that to the test. The tester will pass if within and fail if outside which is very black and white.

I made the enquiry with an opinion on the subject matter and sought some form of clarification some how.

As I said at the beginning take from this what you choose too. :slightly_smiling_face:


If I understand you correctly I don’t think your interpretation of this is the same as mine. If removing one of the cats increases the emissions, then it’s illegal even if the resultant emissions are within the legal limit isn’t it? Removing a cat must surely increase the emissions, all else equal?

There is a qualifying addition to this that says you can replaces two cats with one, but only provided that there is no increase in emissions.

Interesting also that the criterion for legality of an aftermarket exhaust is not whether it breaks the noise regulations but whether it makes more noise than a standard one. If enforced, that would seem to outlaw most aftermarket ‘sports’ exhausts?

Indeed, as I went on to say; the MOT test is not the same thing as Construction and Use regulations.

That difference is true of all sorts of stuff. You might for example fit lamps which don’t meet the lighting regs but will pass MOT because that doesn’t involve any dismantling to check things. So you could have a fresh MOT but if stopped and inspected your car might not be legal. It’s an irrelevant difference for most of us as, well, I can’t speak for anyone else but the number of times my car has been stopped and inspected in the last several decades is nil.

The grey areas are interesting because they raise issues like what if you removed EGR from a Mk.1 MX-5 which came with it? It would still pass MOT emissions because EGR reduces NOx and that isn’t tested in an MOT. It should fail MOT for the missing OEM parts, but likely won’t fail as the tester has to spot they’re removed and it’s not obvious when not all models had it fitted (same year Roadsters don’t). Either way it isn’t legal under the C&U regs.

This thread just goes to show how something that is effectively in B&W can be interpreted differently by different people.

The point is that the MOT station testers use their judgement and as with the responders on this thread - they will all be different.

In the end you are really at the mercy of the MOT station and I am sure we know of ones who are more “liberal” in their interpretation of the rules than others.

In practice, if an aftermarket exhaust meets noise and emissions standards, it should pass… but it is not guaranteed!


Well, I envisaged this to be honest.
I know where I am with it and perhaps others feel the same.
Clearly some don’t but that’s life.

I guess we can all pick bits from a subject and say, what if this or what if that.
Or my car passed an Mot at that station but failed at that one because the sun was out that day and perhaps the tester had a cold or a hangover or his wife shouted at him before going to work or forgot to give him his packed lunch or his kid kicked the ball at his car and dented it! :laughing:
Yes I make light of it, but its human nature.

Perhaps its best for others to comment on different scenarios. :+1: :slightly_smiling_face:

Even nowadays, we are very fortunate regarding MOT inspections. Where I take mine, they had a couple of exchange apprenticeship mechanics from Germany. One of them, when observing the MOT of my highly modified 1.6 NA Eunos, was amazed at what was ‘allowed’ :thinking: Through his boss, he asked if I would take him for a spin once finished on the ramps! I duly obliged and he just could not get his head round all the mods! TUV, etc was/is a lot more restrictive I believe? We are fortunate, as said.