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Catalytic Convertor woes


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Just took my 05 Outback for its MOT and it has failed miserably due to the cats not working.

The tech said baked bean tins would be just as effective.

Strange as my brand new stainless exhaust, complete with new cats, should have been the dogs danglies.

I was having trouble with P0420 error codes but the exhaust guy moved the downstream Oxy sensor a few inches and they disappeared.

I guess he'll have to think again as the exhaust comes with a lifetime garantee.

Pain the **** is having to take another day off work and also having a car with no MOT until he sorts it.

Not happy.

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As it happens I bunged the tester a tenner and he passed it so at least I can use it.

We tried four times after he closed up today to get the cats hot enough on a rolling road and I even took it out for a 20 minute blast before putting it straight on the emissions machine. Still came out way to high for a legit pass. He said he has lots of probs with full flow sports cats like mine.

He even tried taking a plug lead off and running for a minute to get some raw fuel into the system which can bring the cat temps up.

I suppose that would explain the P0420 codes bringing on the CEL lamp every other day.

Spoke to the exhaust bloke and he says that he'll swap them out for new ones FOC if I show him the MOT print out.

Bit of a problem there as that would drop the tester in the !Removed! for passing it.

I'm going to ask him to put his own machine on and take the readings.

I'm sure it will get sorted but it means taking time off work and the exhaust bloke is a 60 mile round trip everytime.

It does sound good though at full chat.

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Oh least got it sorted. And I know what you mean don't want to land someone in the **** after they have helped you out. I'm like that with my tester. He's good with me and has been for years so try not to upset him too much :) Even though the sods rounded of my spacer bolts :( 

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some of the 100 cel sport cats struggle with the mot readings, the 200 cel are more expensive, more restrictive, but perform better


mine has full decat and was borderline with readings, but passed



if high readings is the only problem on my mot, then I'm a happy bunny. what pollutes the atmosphere more - my car that does 4,000 miles a year or my diesel work van that does 30,000 a year?????

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  • 4 weeks later...

OK here is the latest.

Took it back to the exhaust guy and he put it on his tester.

Sure enough it showed high CO readings and also high Lambda readings.

He said that until the Lambda is reading OK he can not say whether the cats are working properly.

So - £320 later and two new lambda sensors fitted I am waiting to go back and get him to test it again.

I'm not convinced it was/is the sensors but he flat refused to do anything until I changed them.

Watch this space and I'll let you know how it pans out.


Somebody also told me that the emissions tester can draw fresh air in through the linked twin outlet pipe so one of them needs to be bunged up to get a proper reading ??? Anybody heard of this ?


Also said that if the gasket between the manifolds and the cats was not replaced it can let in enough Oxygen to give a high Lambda reading without any signs of blowing? Sounds feasible so I'll get some new gaskets as the exhaust place used the old gaskets as far as I know.

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I'm sure the exhaust bloke could get it MOTd every time but I dont want to be tied to them every year with a 60 mile round trip when my mate is half a mile up the road. With the new (genuine Subaru) Lambda sensors fitted he has no argument if the cat is still below efficiency. The Subaru tech said the Lambdas only last on average 60k miles before they get sluggish so as mine has done 120k miles changing the Lambdas wont do any harm. I'll also get him to fit the new gaskets.

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it sounds like your mate is being an !Removed! though, I'd be happy with the 30 mile trip


also, what happens if he's still not happy? you are over £300 out of pocket, what if he 'thinks' it's something else?

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Well  now it has new Lambda sensors fitted I'm going back next week and he can put it on his tester so then we will see if the cats are working.I'm going to get my mate to bung it on his tester before I go so I'll know if the exhaust bloke is trying to pull a fast one.

It sailed through its previous MOT before he did the exhaust so there will be no arguments if it is still not passing.

The engine runs sweet as a nut so its nothing wrong there.

If he still wont do anything about it I'll go to court over it as it is not fit for purpose and not as advertised.

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Update. I got my local mate to run it through on his MOT emissions tester which shows the new Lambdas are all in order but he CO levels are off the scale pointing to a dead cat.

Spoke to the exhaust bloke and mailed him the print out and he is reluctantly going to change out the 100cel sports cats for some 200cel cats next Tuesday.

He still disputes that the cats are at fault but can not argue with the independent print out and the fact that his own tester shows they are way under efficiency in terms of MOT standards.

So another 60 mile round trip, another days lost money, and hopefully it will be sorted as I'll get him to test it before I leave just so everybody is happy.

Watch this space.

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just read your latest reply and had to reread the thread. I had misread who was wanting the lambda replaced  :huh:  (the wife is always nagging me/talking to me when I'm on here. I end up reading things about 5 times or missing bits)


with the lambda replaced and a print out, the exhaust guy hasn't got a leg to stand on. the cat should pass easily for 5 years, it needs replacing.

glad you could get it tested before you go and don't stand for any excuses

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100 cell cats are pants, they need to be really hot to be efficient, mine struggled to pass 1st time on a new cat, weather that is due to the overfueling issue I had, I'll see this year with new map in place

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OK so yesterday I had the cats changed FOC from 100 cel to 400 cel (whatever that means?).

Today after a 30 mile drive I took it to my friendly MOT man and got him to put it on his emissions tester.

The CO reading has dropped from 0.646% to 0.203% which he says is a borderline pass.

The HC has gone from 40ppm to 12ppm which is excellent.

The lambda is 0.998 which is about middle for the acceptable range.


So I'm still a little disappointed as I expected the CO to be less than it is but at least it proves the first cat was just not efficient enough.


My MOT man took the air filter out and the CO reading dropped to 0.199 which is a pass so I now suspect that my engine is running a bit too rich.


I'm going to try and clean the MAF and MAP sensors to see if that makes any difference but does anyone know what causes rich running?


The plugs, air filter, oil, etc was all changed less than a year ago.

I'll pull the plugs and have a look see if there is anything obvious.


I'm determined to get to the bottom of this.


Thanks for you input.

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  • 2 weeks later...

Ok and hello again. I've turned into a techy geek regarding my motor.


I downloaded  a brilliant free program called RomRaider and had a look at my ECU readings via the OBD2 port.

It allows you to see what is happening with all of the various sensors and can also capture data as you drive for examination later.

It was designed for Scoobys and runs on a laptop so if you dont have a copy get one here. www.romraider.com


What it showed was that my MAF sensor was a bit sluggish and only read from 0.8 to 2.1 volts even at full throttle under load.

The internet says it should be reading up to about 4 volts at full throttle for an analogue MAF which mine is.

This makes the ECU think the engine is drawing less air than it really is and it therefore the ECU wants to run lean on the fueling to match the supposed air flow rather than the actual air flow which is much higher than indicated.


However the ECU then gets the O2 sensor(s) telling it to add more fuel to correct the full throttle lean condition.

But this then makes the engine run rich at smaller throttle settings as the fuel adjustment is a "learned" figure that is applied at any throttle setting and only updates every now and then rather than real time.

There is also a real time fueling correction applied by the ECU which either adds or subtracts from the previous correction but in all cases the preference is to run rich rather than lean to prevent engine damage.


Also because there is an apparent lean condition indicated by the O2 sensor the EGR is constantly trying to bring down the combustion temps by feeding dirty exhaust into the inlet manifold at lower throttle settings. This has the effect of slowing combustion and combined with the O2 corrected fueling it shows as a rich exhaust with high CO levels with HC and lambda all OK.


I took the MAF off and it was filthy. Covered in a sooty substance which I can only assume is fine road dirt and oil mist from the K+N filter. I fitted a standard dry air filter. The MAF also measures the intake air temperature which can also have an effect on fuelling requirements. Now these cost a fortune for what they are and I'm mean so.............


I very carefully cleaned the MAF with a tiny paint brush and carb cleaner. Once dried I put it back and went for a run. The MAF voltage now varies between 0.8 and 4.1 volts which is much more responsive and the O2 signals have stopped calling for more fuel so often at high throttle. There is still some fueling correction going on but my internet investigations show that so long as it remains below +/- 5% that is normal. Please be aware that some types of MAF can not be cleaned in this way so do so at your own risk. Check the internet first before going at it with a wire brush.


I have also blanked off the EGR valve and cleaned the MAP sensor. An oil and filter change, new set of NGKs and things seem to be running so much better. It may be my imagination but the car seems so much more responsive (not that it was bad before).


I am waiting to see if the blanked EGR will throw a fault code (P0400 or P0401) but so far so good and at least none of that nasty exhaust is being fed back through my engine no matter what the ECU wants. I have run with blanked or removed EGRs on all of my other vehicles for many years with no issues so do not expect any with my Outback. I did try just unplugging the EGR but it just bought on the CEL and logged an error code P0405.


I'm going to run for a week or two then get my mate to put it back on his emissions tester to see if I have lowered the CO readings.

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Good doings mate, there's loads of info that can be gathered from the likes of rom raider and the subaru select monitor, you should be able to reflash the ecu or make changes to it if it takes your fancy, however I wouldn't recommend it unless you take an image of it before hand

Sent from my SM-G850F using Tapatalk

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Got a P0400 code this morning so blanking the EGR is not so straight forward as on my other motors.

I'm guessing that the ECU is looking for a change in the exhaust via the O2 sensors or a change in manifold pressure via the MAP when it opens the EGR so simple blanking wont work.

I'll pull the EGR valve and give it a good clean and put it back normally then see how things are.

Never mind.

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