MOT CO emissions failure

[Trog_nfs]

Hi chaps,

took my car for its MOT today, and it failed on its emissions.
It only failed on CO though, and only at fast idle.

The results are as follows:

Fast idle Test
Engine speed = 2745 rpm
CO= 0.53
HC = 100
Lambda = 1.020

Second Fast idle test
Engine Speed = 2800 rpm
CO = 0.50
HC = 87
Lambda = 1.020

Natural Idle
Engine Speed = 850 rpm
CO = 0.21

What would cause just the CO to be high on a fast idle?
The catalyst was new last year, but it hasn't had a lambda sensor in a while. Couldn't tell you how old that is, at least 4 years old I would say.

I've done a fault code check, and there's nothing there.
The only thing is is the engine was replaced a couple of months ago, and we didnt do basic settings, and as the engine has been running fine, didnt feel it was necessary.
Would this affect things at all? The idle is a little unsteady, but we are only talking of fluctuations between 850 and 875 rpm.

I found my emissions report for last year:

Fast idle = 2890 rpm
CO level = 0.07
HC = 42 PPM
Lambda = 1.002

Natural idle = 850 rpm
CO level 0.02

Now is it just me or is there a significant difference between all the readings from last year and this year?
The old engine which did the emissions last year died after head gasket failure. I've read that antifreeze can ruin a lambda sensor, and looking at all the results compared with each other, would you say that its the sensor causing the increased emissions?



Thanks
Trog