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So this one has me scratching my head. Granted the car is coming up for it's 18th birthday (don't worry, it's first pint is on me) I am leaning towards the maniverter just getting to the point where it needs to be put out to pasture and replaced.

Got the following emissions results:

Fast Idle
CO - (Limit : 0.2%) (Actual : 0.27%)
HC - (Limit : 200 ppm) (Actual : 115ppm)
Lambda - (Limit : 0.97-1.03) (Actual : 1.001)

Second Fast Idle
CO - (Limit : 0.2%) (Actual : 0.26%)
HC - (Limit : 200 ppm) (Actual : 45ppm)
Lambda - (Limit : 0.97-1.03) (Actual : 0.993)

I am going to give it a thrashing tonight to check the temperature gradient across the catalytic converter to check it's condition as well as checking the sensor readings using an OpCom. What other diagnostics / information would be critical in figuring out where this issue lies?

The lambda reading seems pretty spot on so I would be hesitant to say sensors are the cause though a few years back I did replace the downstream sensor however the upstream hasn't been touched. I just want to avoid the typical Hayne's user approach of firing new parts in it until the problem is resolved, I want to learn how to properly diagnose emissions.
 

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I had a fail on my petrol Modus in Spain and I put in a Spanish equivalent of Redex petrol (I always put Redex diesel in my Combo every fill) and drove it in low gear everywhere until the retest. The numbers dropped like a stone. My petrol Focus in the UK (I hadn't driven it for months) I put Redex petrol in before the MOT and it passed.

Although my Modus also has long layoffs, the ITV (MOT) drive was some 50km and mostly 100km/h roads so I didn't think a blat and some cheap (Redex is £4 or £2 with a Tesco Clubcard) cleaner was the answer, but it was. I don't tend to take the Modus much past 3k revs even on the autovia so it should get you well under the limits.
 
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