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Ford Focus failed MOT on high CO emissions.


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Hi all,

I bought this Ford Focus about a year ago and it's failed it's first MOT under my ownership.

The engine management light has been on for a few months (which I ignored, oops), and it said something about the catalytic converter, I can double check the full error later.

Anyway it's failed the MOT and here are the results...

Fast Idle Test
Engine Speed = 2565
CO = 0.36  (FAIL - MAX OF 0.20)
HC = 115
Lambda = 1.012 

Second Fast Idle Test
Engine Speed = 2710
CO = 0.33  (FAIL - MAX OF 0.20)
HC = 111
Lambda = 1.011 

Natural Idle Test
Engine Speed = 795
CO = 0.30  (PASS - JUST ON MAX LIMIT OF 0.30)


It's been suggested that I need a new catayltic converter fitted (it's a complicated twin one that takes hours to fit). So all in all this is a very expensive option, is there a possiblity it might not even fix the emissions problem?

Is there any common issues with this model of car that have a cheaper solution to the problem?

What about these Cat Cleaner additives I can put in the petrol tank? 


By the  way I don't have any issues with the performance of the car, it drives really well.

Thanks for any help here I appreciate it.


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I can't recall what the readings were on my 1998 Escort (with catalyic converter) but when it failed the MOT on emissions it was the oxygen sensor that was faulty. On this car it screwed into the exhaust a few inches from the manifold and before the catalytic converter. 

The MOT garage was not interested in trying to fix it, it was an old bloke, who has now retired/closed down and he had no modern diagnostic stuff nor knowledge of this modern stuff, and this was before the normal owner like me had an OBD readers.

I took it somewhere else for a diagnostic check. he plugged it in and showed me on the display how long the fuel injectors were staying open on each stroke and said it is much too long.

The  display also showed the voltage from the oxygen sensor and he said it is far too low. He said that in his opinion the excess emissions was caused by the fuel injectors staying open too long and that was because the ECU was getting such a low voltage from the oxygen sensor, so it was no shortening the opening of fuel injectors .

He said this was his opinion but he could not accept liability if I spend money elsewhere on parts/labour and it turned out to be something else. But as he showed me the readings whilst the car was running and explained it to me I felt confident in this being good advice.

I bought a new oxygen sensor and installed it myself and the car was fine (correct emissions and MOT pass). 

I did check the connectors on the oxygen sensor were good before doing this in case it was just a corroded connection or whatever. It was not that. Also the car drove perfectly, it did not feel like an old car with the choke stuck on etc.

Anyway, if an analysis is done of yours (I do not know if cheap OBD readers from eBay etc can give as deep analysis as I describe above), but if an analysis shows the opening of the fuel injectors is long and the oxygen sensor voltage is low then you know there is no point in replacing the catalytic convertor at this stage as the problem is upstream from that.

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the lambda doesnt indicate a sensor issue missfueling. does sound like cats had it.
but as mentioned you have the engine light on. what codes can you remember they were? probably cattalyst efficiency below threashold. p0420-pcm

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9 hours ago, LR2016 said:

What about these Cat Cleaner additives I can put in the petrol tank? 

I have used cataclean after an MOT fail and it got the emissions down from .273 to .110.  The MIL light was still on because I didn't reset it and I continued to run the car for about 400 miles on Shell V power then the MIL went out by itself. That was 3 months ago and it hasn't come back on since. 

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Thanks for all the replies! 

I've found out the error code...

P0420 - Catalyst System Efficiency Below Threshold.

I should also mention that there's no weird smoke coming out of my exhaust at all.


I just watched a video of Terraclean on Youtube and it didn't look like it brought the CO down by much, but it definitely helped their HC.


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  • 4 weeks later...

Hello everyone, just a quick update on this as I hate searching for threads myself and finding the original poster never writes how he got on  :biggrin:

Had to get a new catalytic converter in the end, this got the CO down to 0.02!

Cheers for the replies.

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  • 4 years later...

Hi Lewis, it's been a few years - if you still see this, could I ask you to go even further and say how long that fix lasted?  

I have identical issues, but I'm aware my car's 2.0 Duratec engine is burning oil (IE it gets through it but isn't leaking).  So the catalytic converter may have gone because it's being killed by the exhaust contents... wondering if any of that was true for you too - and how long it all held up for after you replaced the cat?



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