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My 1996 46,000ml MGF vvc has just failed it's Mot test on the exhaust emmisions

Apart for the fitting of a K&N filter and a Pheonix exhaust no tweeking of the enging has taken place and the car saled trough the test last year.

The failure rate was rather high on fas t idle test.

1st fast idle test
@ 2544 rpm the CO level = 8.21% max = 0.30%
HC level = 292 ppm max = 200
lambda = 0.83 max 1.09

Second fast idle test
@ 2982 rpm the CO level = 4.39% max = 0.30%
HC level = 363 ppm max = 200
lambda = 0.95 max = 1.09
Natural idle test
@ 866 rpm CO level = 0.72% max = 0.50

Can anyone offer advice on how I can remedy this and how much will it cost ?.. I have only 7 days in which to get it sorted to qualify for a re test without having to pay another 37.00

Many thanks in anticipation of help.

frank whitehead

could it be

faulty O2 sensor?
new spark plugs required?
re-set ECU?

My 97 VVC failed and it was down to a faulty lamda sensor in the exhaust manifold.

Easy and cheap if it unscrews, expensive, if like mine, they wreck the manifold.

...hope the cat is not worn out too...

Maybe I am looking too simplistically at the results - but doesn't the high hydrocarbon and carbon monoxide level indicate that the catalyst is dead as David fears? :o(
Rob Bell

The cat would be my first suspicion too, with the age of the car too, it sounds about right (although you haven't done that many miles...?)

You could probably pick one up from a breakers quite cheaply, better than paying for a new one to find out its something else.


Steve Childs

How much would a new cat set me back folks ?
frank whitehead

They can vary in price. On a Rover I had they varied from just under £100 to over £300. I never got a quote form the local Rover dealer either. BTW these prices were to fit myself.
I know it is not an F but it does illustrate that you should shop around.
Cecil Kimber

I just had to have a new cat, just to be amazed at the price they would give me i rang the dealer, just for the cat, they quoted me £735 inc VAT , without fitting, i told them i'd back to them when hell frezzes over.

Rang around a few places, and you can get them as cheap as £120 , but these are ceramic, and not good from what i've been told.

Anyway, i got a 2 year guranteed stainless steel cat for £180, bought a 2nd hand exhuast on this board for£50, and had them all fitted, and paid for £299 at an independent garage.


Standard MOT test procedures often leads to fail results, why, because some cats are choked up and need to be 'lit'.

If the tester sees a failure first time round then the stabilisation time before the next test starts can be used to rev the engine a number of times up to 5000rpm plus and backing off. The emissions will jump sky high as the system goes open loop, but the surge of flow, heat and fuel will usually 'light' up the cat and see it come within scope for the test.

This is not a cure all, but I see it quite often, perhaps 10% of all tests I view. A good thrash before a test is another good policy.

Roger Parker

Cheers Rog ,, the car was prity cold when I left it at the garage for MOT. However..........

The dealer ship has it now . Well they have had it for the last two days . Dealer Called today to say they still carn't find any fault yet the emmissions are sky high and it is possibly the oxegene sensor cost £150 notes but their not sure. Been advise not to expect car back until middle of next week. Why do they build these cars so complex if their own mechanics carn't bloody fix them.

frank whitehead

Frank, I fear you are being taken for a ride. These cars are not complex compared with most cars on the market today, and high emissions is pretty straightforward to diagnose. As stated by the (largely) amateurs on this BBS, it can only be 1 of a very few items at fault. The ECU can be swapped temporarily with another car, the sensor should be a stock item so again can be swapped out to see the effect. 30 minutes work at most, what are they playing at ?

Cat / ECU / O2 sensor

One of the three. Most likely the catalyst IMO given the age of the car. Rog's tip is valid and a strong possibility.

Scarlet Fever

Hi all,
one small independent garage not far away from me have repeated sucsess in "rejuvenating" old cat´s. They hang the cat vertically and uses a hughe acetylene / oxygene torch in order to heat up and by altering the acetylene/oxygene mixture thru the cat "burn away" any trapped soot etc. A bit brutal - but it is said to be a treat to get thru MoT. And from those who have older cars that have gone thu this treatement it seems to work OK - HC and CO seems to stay down.

BR , Carl.

Have you any more information on "burning off" cats. Is the mixture passing through oxygen rich and is it lit? How long or how hot do they get the cat up to and is it heated from the outside or by directing the flame through the cat? Sounds very interesting and I'd like to know more.

The garage still have the car.......!!!!!!!!!! they suspect it is the sensor in the manifold so they tried to remove it, whilst doing so they ruined the thread in the mainifold thus this has now been removed and will need tapping..... When I visited the garage today the mechanic said he was pritty sure it was this sensor.... I repiled I don't want pritty sure I want dame sure / positive. he stood there with Blank look and no responce. I went on to say I don't see why I should have to pay for somithing they had dammaged. ( Hence now they have whisked off the old manifold to an engineer to re thread the sensor port. ( no doubt the cost of this will be in my bill )

Brian ( above )from bucks mentioned that the garage ruinined his manifold ....My bill is alraedy standing @ verbal 195+Vat and this garaged ( MG /Rover Approved Speed six NR Wrexham) are not filling me with any confidence. Can anyone advise of any recourse if all this doesn't solve the emission problem? My other fear is that they may fix this and caurse some otheer problem in the process and at a latter stag denied any reposibility.

Do I sound bitter ?

Yours becoming increasingly more frustrated ( not had car now for 4 days and counting )

frank whitehead

Yup! Another wrecked manifold coming up.

I was lucky in that Priests of Chesham in Bucks put me onto Rover's customer support people and they paid 75% of the material cost as a guesture of goodwill. The car was only just out of its 3 year extended warranty.

Hi John,
haven´t seen this myself and it seems beeing a bit of a trade secret ;) What I been told they strip the cat of any clamped on heatshields and hang it vertically. The main heating is on the outside. Then a short test with the flame thru the cat and by some means (the smoke ?) they can determine if it is up to temp. If so the flame is going thru the cat - what´s not told is if it is oxygene OR acetylene -rich.. IMO a slight oxygene -rich flame would burn away any deposits. But too much would possibly melt and damage the cat. Of course this treat is only sucessful on a mechanically healthy cat !

As for Franks problem ; I dont really understand why the garage cant determine if the O2 sensor is shot or not in situ ! If there is enough span on the voltage readings given = OK, if too narrow span = Dead. After such a test it might be worthwile to start undoing the O2-sensor. Trying that without heating with a flame leads nearly always to destruction of the threads or even worse a bent mainfold. Also take in consideration the stresses to the head when trying to undo the sensor with a long spanner...
Hope You get it sorted without too much outlay,

BR, Carl.
Carl Blom

Oh dear, unfortunatly I used the word "sh#t" when I described what to burn away from the cat ! The punishment is to log in as a new member and forever linked to USA ? Any advise how to become the old "Carl, Sweden" again ?

BR, Carl.
Carl Blom

We forgive you Carl!

>>As for Franks problem ; I dont really understand why the garage cant determine if the O2 sensor is shot or not in situ ! If there is enough span on the voltage readings given = OK, if too narrow span = Dead. After such a test it might be worthwile to start undoing the O2-sensor.<<
I completely agree - can modern technicians use a DVM / 'scope ? They should be trained for this sort of problem determination - it's a lot easier than undoing the sensor !
Then again, if it's a MGR dealer, I would imaging TestBook would tell them conclusively of the failure.


I have the car back........ ! with a PASS on the emmision readings... It was the O2 sensor. Cost £270 inc Vat...charge for the sensor alone = £124.44 +Vat

Thanks for all your help and advice..

yours, skint for the next month or two

frank whitehead

Try a run up the road to light the cat up first. then try a bottle of FORTE GAS TREATMENT...good stuff. if this doesnt work the car should now be put on T4 to see if the oxy sensor is swinging if it is not it is a good chance it is the oxy sensor. the only other thing that controls the oxy sensor is the ECU. if the oxy sensor is swinging go for a cat.


This thread was discussed between 25/06/2002 and 03/07/2002

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