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MG MGF Technical - MOT emissions failure 98 VVC

My F has just failed it's MOT ( CO2 level 3.31% spec.0.30%) they say it may be CO2 sensor failure or the catalitic converter or both. Any comments on past experiances and how difficult it is to fit a new sensor would be welcome.I believe I'd need a special spanner ( where can I get one) and I've heard there is a possibility of the thead in the exhaust manifold stripping and then needing a new manifold.Hope you can help

If you heat the boss up that the sensor screws in to with a blow touch it should came out without problem, you only need a normal spanner but cant remember what size 19mm ish, see diagram for testing information, if the cat has gone your normally lose power from engine, odds on it will be the sensor

Alex Sturgeon

If you tear the thread when removing the sensor, then it is easily repaired by fitting a 16mm Spark Plug Helicoil insert. Any good engine shop can do it.
First check the wiring and clean the connector.
Geoff F.
Geoff Farthing

You need a 22mm spanner. As Geoff said I stripped the thread and had to get are placement manifold. The helicoil kit cost over 70 to buy and the local machine shop wanted 92 to do the job. I don't have the slotted socket made for the job but I don't think there is enough space fot it anyway. I used an offset ring spanner and cut a slot in it to pass the wiring through when I fitted the new sensor which I bought from cats2go, very reasonable price.
Ken Waring

I have a 97 VVC and found it easier to just drop the manifold. Plenty of WD40 on the threads break top off sensor then using a socket and bar try to tighten first then undo. Took about an hour.
Tony T
t thompson

I had a go at removing sensor. soaked the area in WD40 then ran engine until it was hot, I got it to move half a turn then it locked solid I guess the threads shot.Have booked it in to garage to be done. The design guys who put service items in inaccessable places should be made to work on their abortions for a year. Thanks for your help anyway


Before you go down the route of changing the Lambda sensor its worth checking the following:-

air filter is clean - unlikely but simple to check so worth a look.

No vacuum leaks/split pipes - you'd be amazed at the difference a small split can make, it can be linked to poor running at idle.

Map sensor (not sure if this is remote on the MGF but if it is then check the vacuum pipe to it for the same reason as above.

Operation of the lambda sensor. (Output voltage is correct). There are different types of lambda sensor, yours is probably a self generating sensor (i.e it produces a voltage. If it is then you can test it by connecting across the signal wire with a multimeter on the 2v setting (any higher setting and it won't read properly). The sensor should continually change between 0.1 & 0.9 v when the car is fully warm. If it sticks or is above or below this sensor then it's faulty. Of course if it continually changes between these limits it doesn't mean that it works properly, it may still be out of calibration/contaminated.

Coolant temperature sensor. Check the resistance when the car is warm, some cars (again not sure about the MGF) have two sensors, one does the gauge the other does the ECU so even if the gauge reads right the ECU may still think the engine is cold so it overfuels. You should be able to get the correct resistance readings for these sensors by searching the internet but if you get stuck let me know and I will have a look for you to see if they are on my Autodata disc.

Happy hunting!


Andrew Smith
A D Smith

This thread was discussed between 17/11/2008 and 27/11/2008

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