CAT replacement - What do you think?

  1. My model of MX-5 is: MX5 MK2.5
  2. I’m based near: Sunderland
  3. I’m looking for technical help or recommendations on: CAT/CAT Replacement


I have a NBFL '04 and I have recently thought about replacing the CAT, but I’d like to know a little more about the CATs themselves. My reason for replacement is to actually put the original in storage (if it is original), as I’ve read upon the cutting and theft of original cats at least from Jap cars because of the rare elements and prices for these elements.

I guess these are the most important questions to ask:

  • Would I still pass an MOT with an aftermarket CAT?
  • How effective are replacement CATs over OEM in terms of performance and maintaining the cars original character?

If anyone has any indications on prices for potential sale of my OEM cat, I’d like to know more.



I’d be interested to see the replies you receive…

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The reason or mk 2.5 converters are so valuable is that they were made in the day when precious metals were not as valuable as they are today so they made the core out of a solid lump of the stuff. Nowadays they use a cheap core with just a coating of the expensive stuff. The active surface area remains the same so it still does the same job so compliance will be no problem if it is a reputable replacement & not imported any old rubbish from outer Mongolia or somewhere. A catalyst itself doesn’t actually wear out so a coating is just fine. They only stop working properly if something contaminates it.



I am a little confused? Clearly you are aware of the spate of catalytic converter thefts occuring and that MX5 cars are a particular target. Only today I had a call from a well known MX5 Specialist where 10 converters, including customers car parts were stolen.
Original MK2/MK2.5 cats are still red hot targets although I see the price of Rhodium has dropped around 20% in a couple of weeks.
Take control before someone else does. There are a lot of people who would have you believe that aftermarket cats are ineffective and your car will never pass an MOT with one fitted. These people are ill informed idiots and to be honest not sure what their angle can possibly be other than spouting confused nonsense.
If any one of these people has an alternative strategy to mitigate against original part theft please come forward with a reasoned explanation.
For my own part I run a 1997 MK1 1.6 car fitted with an aftermarket cat, supplied by the manufacturer I retail parts from in 2003. It still gets the car through MOTs. The reason for this is that the car has a reasonable level of hydrocarbons(soot) at less than 20 PPM, demonstrating that it is running reasonably well. The MOT pass is 200 PPM and should not be used as any sort of guide. Figures anywhere near 60 PPM are excessive and at that level a warranty return would be rejected for good reason.
Your car has the 1.8 VVT engine, so I would expect the emmissions to be well within acceptability.


Everything as rhino666.
Nothing wrong whatsoever with aftermarket ones, (why on earth would they be?).
As a bit of information, “Powerflow” (Branches nationally) will match a CAT to your car regarding size, fitment, engine power output etc etc.
I spoke with my local chap and they have a very good database to suit.
As an example, the NC2 2.0 centre one has an odd shape regarding inlet/outlet and thus pipe fittings. He found one to suit that would have matched the original.
Not saying they can help of course, but nothing to loose.

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We had the CAT cut off our 2007 Prius. The drive home was loud, but not too far. Aftermarket CAT fitted by our regular garage and no problems since with the MOT emissions.
For some models of cars there are anti theft devices that can be fitted but I don’t know if there is one for the Mazda MX5’s.

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Unfortunately an anti theft device will just highlight the fact that you are protecting a valuable cat. The thieves may be deterred whilst there are easier targets but inevitably the cat will be stolen and further damage done to the anti theft device and likely your car when they have to wield their cutting tools in anger to get it off.
As these thieves are just as happy in a Tesco car park in broad daylight as they are at your home in darkness, do yourself a big favour and duck this issue before it is one.
The only sensible suggestion I am aware of is take the original cat off and sell or store; replace with an aftermarket cat.
If anyone can suggest another way that makes any sense, please post and lets discuss.
I have had daily calls about theft all this year. The problem is not going away. Hearing from a colleague in the trade yesterday, not the first MX5 specialist to be targetted by the way, is upsetting. They called me trying to source a new OE cat for a showroom condition MK2.5 that was included in the theft. Hopefully I have persuaded them to fit aftermarket and compensate the owner in cash for the shortfall.


Thanks for all the reply’s. My plan is to replace it with an Aftermarket Cat and sell my OEM one.

However, one factor I didn’t realise was that the CAT and the Manifold are one piece (Correct me if I’m wrong) which is a bit of a pain. However I am sure the cost of my original OEM cat would cover that.

Since I am getting the Malian Backbox, It would probably be best to purchase the Manifold and Spots Cat single piece here:
Mazda MX5 Mk2.5 1.8 VVT EL Tubular Manifold + Sports Cat Performance Exhaust – Malian Exhausts

No, on your MK2.5, assuming you still have the OE system, the part is cat and front pipe. The front pipe flange bolts to the manifold, a separate part as standard.
If you take a look at the link I sent you, the picture is clear and part direct fit for the OE one to be removed.
That part on your link looks good but has a low volume sports cat, so around two thirds of the catalytic surface on a smaller core. This will be detrimental to catalytic effect and will certainly add a lot of noise and resonance to your exhaust system.

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As Rhino mentioned, the cat is its own section. You have the manifold, which ends between the back of the gearbox and the nearside front wheel, then the cat, then the centre section, then the back box section. The join between cat and centre section is the most obvious one under the middle of your car.

One of my recent posts covers some of the pain points I had in removing my cat to sell it for this exact reason.

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Hi thanks for the reply’s again!

Is there any way to get a price for my CAT whilst also finding out what CAT it is?

I have the car on a jack and recorded a small video of the cat itself but I have no idea how to identify it. All I know is my engine is a BP-Z3.

Video here:!AkzIqJedKxU8hNJCA9XsjPhzuGA3eg?e=J63Veh

You could give Jack a call on 01702601115 as he might be interested and give you a price he is based in Southend. One of the other members used him and was please with the money he received.

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Thanks but its a 12 hour drive there and back for me I think id need somewhere a little closer (Im in the North East)

I wonder if the Mazdadudes garage closer to me would buy it :thinking: