Precat Information

Whilst the MR2 Roadster has very few flaws as either a fun cheap sports car or as a precision tool to hit the track with, there is one thing that we as a Club feel every owner should be aware of, and that is the pre-catalytic converters (or pre-cats for short). There have been a huge amount of questions on here since the forum began regarding these, and this thread is here to hopefully answer any and all questions that have cropped up about the pre-cats, as well as dispel some myths about them.

*Please note: The Club neither encourages nor advocates the interference with emissions equipment on any motor vehicle, and we take no responsibility for any action taken by any person as a result of reading this article. All text and pictures here are for information purposes only.*

What is a pre-cat?
To put it quite simply, the pre-cats sit before the main catalytic converter in the exhaust system and help to keep the harmful emissions as low as possible for a short period after you start the car up. Of course, there is slightly more to it than that…

The main catalytic converter in the Mk3 works best at converting the harmful compounds contained within the exhaust gas at high temperatures: However, since the engine takes a while to heat up to it’s optimum running temperature, there is a time when a great deal of harmful emissions are allowed to simply pass straight through the cat and are dispersed into the air. Toyota obviously wanted to keep these emissions to an absolute minimum to enable the car to be classed as a ULEV (Ultra Low Emissions Vehicle) to allow the Mk3 to be sold in California (they have practically the most stringent rules on car emissions anywhere in the world there!), so between the engine and main cat they placed two pre-cats contained within the main manifold itself. The manifold itself looks like this:


The four headers run into the two chambers containing the pre-cats, and then they’re passed onto the main cat to let it do its job. The pre-cats are made from a ceramic material, which whilst excellent at absorbing the noxious gasses at low temperatures, is also highly brittle…

Why are we worried about them?
As stated above, the pre-cats themselves are not the strongest material known to man, and they have been known to break down and enter the engine, causing serious damage to the internals. When this sort of damage has occurred, you are almost certainly looking at needing a new engine.

Woah, wait a minute! How can the pre-cat get back into the engine: Surely the exhaust flow pushes it all out?
True to a certain extent, but here’s the clever bit…

The 1ZZ-FE engine (Toyota’s designation for the engine inside the MR2 Roadster) is a very clever piece of kit, and arguably its main party piece is the VVTi, or Variable Valve Timing Intelligent. This increases engine response all over the rev range by altering the timing of the cams, allowing for differing amounts of valve overlap in order to give great low-down torque as well as good top-end power. The 1ZZ also uses it’s VVT to perform EGR (Exhaust Gas Recirculation) functions without the aid of a specific valve like other cars. Under certain operating conditions (usually steady cruise) the cams are timed to scavenge some exhaust gas back into the cylinders, as a way of reducing the high hydrocarbon emissions that modern petrol engines generate at certain times.

Unfortunately, when you combine this with some very sharp ceramic pre-cat particles, you can imagine what happens: The pre-cats start breaking down, and get dropped into the main cat which then causes excessive pressure, leading to oil blow-by in the engine. When the VVTi kicks in, the pre-cats are sucked back in and scratch and score the cylinder walls, leading to more oil passing by the piston rings and being burnt off without you even realising it. No oil in an engine leads to massive failure as every moving part grinds against metal, and in short you end up with a practically useless engine. When this happens the situation is compounded by the fact that hot oil is now allowed to drip directly onto the pre-cats and break them down even quicker, which in turn allows large chunks to block the main cat even more, which then stops any smaller pre-cat material escaping at all and sucks even more back into the engine to cause even more damage… A vicious circle of the very worst kind.

Some common symptoms of pre-cat failure are extreme oil loss, very noticeable lack of power all the way through the rev range, and horrible noises coming from your engine bay. Essentially, if you’ve got any of these problems and they are directly related to pre-cat loss, then it’s too late. Even the oil warning light won’t save you here, as by the time it comes on there’s almost zero oil left in the engine anyway.

For more information on how an engine works in general, please click here for a link to

But I’ve read elsewhere that the pre-cats themselves are fine, it the piston rings which are the weakness…
This is where we come across a real conundrum, and a question to which no-one has a definitive answer. It’s true that on very early MK3s there was a known problem with the piston rings themselves on a 1ZZ, and Toyota issued a technical document to the dealers around the world stating as such. They also changed the design of the piston rings for the facelift version of the Roadster, which became available in 2003.

Now whether it’s a case of the piston rings failing, oil dripping onto the pre-cats and breaking them up, or the pre-cats self destructing and taking the piston rings with them, we just don’t know. All we do know for certain is that whilst you can’t take the piston rings out of the engine, you can remove the pre-cats from the manifold. No pre-cats = Nothing to get sucked back into the engine.

Okay, so the pre-cats are obviously a bad thing, but what can I do about it? Is there any way to tell if they’re okay on my car?
There is only one sure way of telling, and that it to remove the entire manifold and check both the top and bottoms of the pre-cats for any signs of damage. This is the only 100% way.

I’m not very mechanically minded, so is there another way? Even if it’s not 100%?
Yup, and this is the way 99% of people do it (myself included). It’s very simple, and requires nothing more than a 22mm O2 sensor removal socket (Available from here for one, but you can get them at many other places as well, this is just an example), a can of PlusGas or similar penetrating oil (WD40 will do at a push, but it’s a lot easier with the PlusGas), and a torch.


The picture above shows the heatshield which covers the manifold itself, and is how your car looks when you open the engine bay. Coming out of either side of the heatshield are the O2 sensors, which need to be removed to see the pre-cats from the top only.

1. Get the engine nice and warm first, it’ll make this job a lot easier!

2. Spray the PlusGas liberally onto the joint where the O2 sensor meets the manifold. Leave for 10 minutes, then spray it again. You cannot use enough of this stuff, trust me! Don’t worry about the steam coming off; it’s not doing anything any harm.

3. Being very careful not to burn yourself on the heat shield, use the O2 socket to remove the sensors, Unplug them first from the plastic clip (it’s a simple push-tab-and-release connection), and make sure you turn them anti-clockwise. If you have an older vehicle, you may find that these are very stubborn, but do persevere and don’t be afraid to give it a little elbow-grease!

4. Pull the sensor out of the socket and place carefully on the floor, away from your feet. You don’t really want to tread on it now you’ve done the hard part, do you?!

5. Take the torch and shine it into the holes. You’re looking for a completely solid honeycomb matrix with no cracks or large holes in it, like this:


6. When you’ve finished checking (and hopefully found that they’re still intact), simply screw the O2 sensor back in and nip it up with the socket. Oh, and you may want to plug it back in too. :wink:

My pre-cats look fine! I’m safe! *dances*
Not quite: They’re still very fragile, and remember you can’t see the bottom of the matrix from that angle either. All this means is that your engine is still fine and you’re not in any immediate danger of the pre-cats failing.

Oh, okay. So what’s the next step then?
The only 100% sure way to protect your engine is total removal of the pre-cats from the manifold. This isn’t a particularly hard job, but it is more involved than simply removing the sensors.

Below is a complete set of instructions to do this, written by Grant (GSB). Do it his way and you won’t go far wrong. Many thanks to Grant for producing such a wonderful and complete guide to this.

GSB wrote: Heres how I removed the cats…Note: Clearly, neither I nor MR2-ROC can condone you ripping lumps of emmisions control equipment out of your car. Its safe to say the the warranty on my manifold at least and probably my main cat as well have now been well and truly voided, so the same will go for you. Basically, if you decide to go down this route as I have, you’re on your own… On the plus side, if its not in there, it cant break. :wink:

Toolkit required

Trolley Jack
Axle Stands
10mm spanner
10mm socket on 6″ extesnion
12mm spanner
12mm socket and various extensions
14mm socket
Long flat bladed screwdriver
High pressure water or air supply
Large vocabulary of swear words

In order to remove the precats, you first have to remove the exhaust manifold from the car, Care should be taken to apply penetrating oil to the various nuts and bolts some time before undoing them

1/ Jack up the rear of the car and place on axle stands.
2/ Remove the splash guard from under the rear bumper
3/ Remove 3 bolts from the forward splash guard to let it hang down.
4/ Remove the 3 14mm nuts that secure the main cat pipe to the manifold

In the engine bay;

5/ Using a 22mm O2 sensor socket, remove the 2 O2 sensors from the manifold.
6/ Remove the 4 bolts securing the manifold upper heat shield
7/ Remove the 2 bolts that secure the lower part of the manifold to the engine block. These are ‘behind’ the manifold and not readily visible. ! of them is 12mm, the other 14mm.
8/ Remove the 5 nuts securing the manifold to the cylinder head.

The manifold can now be lifted out of the top of the engine bay.

Decatting the precats…

Actually getting the precats out is pretty simple, here’s what mine looked like from above and below before I started, you can see some of the degradation thats taken place in the first photo:

And here are the surgeons tools: :wink:

Removal is simply a case of attacking the precat matrix with hammer and screwdriver until its broken up into lumps small enough to be able to get them out through the lower exhaust port. The ceramic material gives up very easily, it only takes 1 or 2 taps on the screwdriver to do this… :shock:

After digging around for a while you will also expose the glass fibre that surrounds and supports the cat matrix, this has to come out too.


Once out you’ll have a big pile of very useless, but rather expensive catalytic material left over. I beleive some places do recycle this stuff to reclaim the precious metal content, so if your now feeling guilty about your effect on the environment, this could be a way to ease your concience :wink:

Once its all out, you’ll be left with an empty manifiold like this;

You now need to clean it, as there is still an awful lot of potentially damaging dust and particles left inside. I would recommend high pressure water like a jet wash or an airline for this, as an chemical residue from solvent cleaners may have a damaging effect on the O2 sensors.

Once clean and dry, re-building is simple the reverse of the process used to take the manifold out in the first place.

Happily, I’ve not noticed any increase in noise level from removing these. Performance certainly isnt any worse, and in fact the car may have benefitted in the form of slightly increased torque, but I cant say for sure… What is certain though, is that the pre-cats are going to have a hard time damaging my engine from the workshop bin, and I’m a lot happier now that these ticking time bombs are not a problem…MOT tests wont be a problem, as I still have the main cat in place, which is more than capable of doing the work. I dont have a ULEV car anymore, but I do have one that with a bit of luck will last a bit longer.

Alternatively, some owners have found some garages that will do this for you, for a price obviously.


Please note the above list is not total: If you know somewhere that offers this service, then go ahead and post it up for fellow members to know about!

Hmm, I don’t really fancy doing this to my car: Are there any other alternatives?
Certainly, although they obviously cost a bit more. The obvious way is to change the stock manifold for an aftermarket item, and you can’t really go wrong here as there are no aftermarket manifolds that include the pre-cats. The cheaper options are to buy a performance manifold from either TRD (Toyota’s own tuning brand), PPE or Top Secret. Other manifolds are available, but these are the most common ones and should cost you in the region of £500 each. These manifolds also come with the benefit of extra performance, however don’t forget to inform your insurance company if you fit one!

Another alternative is Che’s Manifold, which has been designed and manufactured by a chap over in the US who posts on SpyderChat. (Click here for the thread on SC). This hasn’t been dyno tested to prove any performance gains, but then again it’s by far the cheapest way to get a new pre-cat free manifold.

There is another way to replace your manifold, but it does get considerably more expensive… Turbo conversion. The Hass, C2, PE and Top Secret kits do not include any pre-cats in their design, whilst the TTE kit’s pre-cats are of the metal type, and are far more resistant to ever breaking. Far more resistant as in ‘will not unless you attack them for a good few hours with a large hammer’. Obviously this isn’t really a solution for most people due to the cost involved, but it’s a fair point to mention.

Okay, so I’ve decided to remove my pre-cats the old fashioned way. What about the MOT? Will I still pass it?
Whilst we cannot say for 100% certainty that your car will pass MOT after pre-cat removal, we do know of a good few members on here whose cars have passed MOT with flying colours, and some more than once. As long as the engine is nice and warm before the test commences, the stock cat is more than capable of filtering out any nasties from the exhaust gas. As stated above, the pre-cats only work for a short period from cold anyway.

I’ve got a very low-mileage car, does any of this really apply to me?
This information should be heeded by every MR2 Roadster owner, regardless of model year/mileage done. To quote my own personal experience, on my 2003 Roadster (after the piston ring re-design), I checked my pre-cats after just 15k miles, and they had started to disintegrate. Luckily I caught them in time, but this just goes to show that it’s not only high mileage/older cars that can suffer from pre-cat breakdown.

Granted, there is evidence that this happens more often on older cars, and as a general rule of thumb it seems that most cases occur around 30-40k miles. However, that doesn’t mean that you should be waiting that long before you start checking the pre-cats: Remember, as soon as they start to go then you’ve more or less had it, and by the time you notice the symptoms it’s usually too late…

As a footnote, when I discovered my own pre-cats had started to degrade, I made Toyota replace my whole manifold for a new one. After 3 days of it being on I gutted it, although not before noticing that the brand new pre-cats had signs of damage to them as well. I’m not trying to state anything in particular here, merely making an observation.

Argh, I’ve caught it too late! My pre-cats are gone and my engine is done for, what do I do from here?!
First off, don’t panic! Although the Roadster is a relatively rare vehicle in the Toyota line-up, the engine it uses isn’t: It’s found in the Celica, Avensis and the Corolla, so if you need to source a new engine it’s not going to be a hard job finding one and at a decent price (from £500 for a high-mileage one to £1000+ for a slightly more recent one). All engines aren’t 100% identical though: You may need different wiring parts and engine mounts fabricating, but any garage worth it’s salt should be able to do this for you.

The other alternative is to repair your old engine. To do this, you will need:

New main catalytic converter
New manifold (stock or aftermarket)

…parts-wise at the very minimum, along with a full engine rebuild to sort out whatever damage may have been caused (not all engines will break in exactly the same way, due to the normal fluctuations in the building process), although you can certainly include cylinder re-lining as part of that.

Total cost of this will vary from garage-to-garage, but you don’t need to go to a Toyota main dealer to do this: All you really need is a garage you can trust, and there’s literally hundreds of quality garages that can do this kind of work all over the country. Ask around here, your friends, work colleagues, and any other places you can think of to get a recommendation for you. Remember, it’s your pride and joy we’re talking about here, so the cheapest place might not necessarily be the best.

*NOTE* If this happens within your car’s warranty period (i.e. first three years), then you need to take your car straight back to your Toyota dealer. The law is on your side in this, so don’t be afraid to use it.

So there you go, that’s just about everything you need to know regarding the Roadster and the pre-cats. This thread isn’t meant to scare you in any way or put you off owning what is one of the finest driver’s cars ever built, it’s just information and a way of combining the knowledge contained in this forum into a single thread to save you all searching. Think about how many Roadsters Toyota have built, and how many pre-cat related engine deaths we know of… It’s a tiny fraction. Toyota build hugely reliable cars, and the MR2 is no exception: The pre-cat issues mentioned above are merely what we as enthusiasts have noted over a few years since the release of the car, and in no way are any slight on Toyota’s ability to mass-produce an affordable convertible. Every car ever built has issues, and we are fortunate that the Roadster has so little to worry about. We just want you to be able to enjoy many years of care-free ownership, and every mile in this little car is one that should be enjoyed with a smile on your face.

This documentation in no way replaces the Toyota MR2 Repair Manuals. The purpose of this content is only to provide supplementary information to fellow MR2 enthusiasts. Midship Runabout and its contributing authors will not be held responsible for any injury or damages that may occur as the result of practicing any of the methods or procedures described within this website. Article and photo submissions are property of the contributing author.

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1 Response

  1. mike3 says:

    I removed the precat material just as you said. I also replaced all 3 I2 sensors. Now I am getting error codes po125 and po174 any thoughts?

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