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MG MGF Technical - Emission Test

Today my MGF1,8i -97 fail the Swedish MoT emission test:

CO=6,8% HC=500ppm Lambda=0,75

Any suggestions?

MF Fällström

Those figures are pretty high - looks as though the cat wasn't doing its job for what ever reason.

Two possibilities:
1. the cat wasn't properly up to temperature when the test was performed (or it might be fouled up with carbon deposits - depends on how the car was used prior to the test)
2. the catalyst has failed.

The latter is expensive to rectify - but happily doesn't happen all that often. Is there a rattling noise from the exhaust system that may indicate that the matrix inside the cat has broken up?

The cheapest option if to take the car for a quick blast in the countryside. Once the engine is warm, accelerate through the gears, taking the engine revs to the red line. 10-20 minutes should be enough. Then retest soon after. If the catalyst was fouled up, this should clear all the unwant gunk.

Fingers crossed that the cheap option works!
Rob Bell

Tanks. I will try the countryside blast and keep my fingers crossed.
MF Fällström

check that the labda sensor is connected by its wires . Not the most easy place to get at ,but on a ramp it can be checked from underneath.If you have axcess to a multimeter (digital voltmeter) an easy measurement will tell if the sensor is OK.
Description where to measure will follow if needed....
Also the usual contact problems on other sensors (coolant etc.) will show up in bad emissions. But most probably a lambda-sensor related problem !
BR, Carl.
Carl Blom

Definitely a catalyst problem. I got mine back today with 0.01% CO (0.2% pass level) and 28ppm HC (200ppm pass level)

The UK lambda pass level is 0.97 - 1.03


I'm sure someone will be along to correct me if I'm wrong - but those figures look pretty high even for a car without a catalyst?

The presence of high levels of carbon monoxide and unburnt hydrocarbons shows incomplete combustion - either over fueling or insufficient air. In which case you probably get quite a lot of soot, which would foul up the catalyst, as Rob describes. Taking the car for a blast might clean off the deposits long enough to get through the MOT, but I think I would still want to check the tuning.

Possible that a fault with the lambda sensor, as Carl describes, would lead to the engine management system being confused and over fueling?
Mark Roper

Sounds like time to give the air filter a good clean!
Steve Ratledge

My money is with Carl on the lambda sensor.

With luck it may just be the wiring - mine had broken through rubbing against exhaust manifold heat shield.

Replacing the sensor itself is a bit of a challenge.

John Thomas

Tank you all. I’m going to check the lambda sensor and then go fore a blast.
Carl, what should a measure on the sensor?
MF Fällström

go to the "Archive " and look for "CO/lambda". You will get a thread from 7/11-01 where further down I do describe where to attach a voltmeter to measure lamda value.
As noted a healthy sensor will fluctate between approx. 0,2 to 0,9 Volt several times/ second when using the throttle. A defect one will be almost stuck around 0,4-0,6 Volt regardless of engine revs.
Well worth cheching before someone starts to try to remove the sensor. Such work usually means that the whole thread on the downpipe gets damaged = both new sensor AND mainfold ;O(

BR, Carl.
Carl Blom

Carl, your *how to* is aswell on

Dieter K.

Yeah, won't be a catalyst failure. You could remove the cat fully and let lower readings. A dirty air filter wouldn't make readings that high either. A lambda sensor is the most likely part to cause this, chaeck it and make sure it's switching as it should. Lucas do universal 4 wire sensors that cost about £30, you just have to remove the plug from the original sensor and crimp to the new.
Steve Tyler

Wow !! My MGB GT didn't have emissions that bad. If the cat's not rattling then it is more than likely the sensor - as it's fairly exposed it may just be corroded connectors, give it a quick squirt of WD40.


Andy damaged the manifold boss thread before the Treffen and fitted a new manifold. He recovered the old manifold and considers that he probably could have fitted a Helicoil. Dieter tells us that it is M18 x 1,5.
MGR Sensor is £120. Ouch.
Dieter gives Bosch 0258 003 229 for Mk1 to 522572

Geoff F.
G. Farthing


I nearly passed the UK emissions without a cat on at all! Yeah I know from working at Volvo that in Sweden the tests are more stringent but try the following, clean the sensor connectors, and then take the car for a good long run. The take the retest, I presume that the engine is running ok and not too rich.

If you need to get a new cat they are frequently available on


This thread was discussed between 07/09/2004 and 10/09/2004

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