Emissions - I'm at a loss!

Hi there,

I'm writing in the hope that someone might be able to shed some light on the problem I have:

I have a '96 Megane RT and I've been at the NCT (that's the National Car Test in Ireland) four times since November and every time it failed on the emissions. To combat the problem, I have had:

- a new cat fixed
- a new lambda sensor fixed
- petrol tank cleaning liquid put in the tank
- a new back box installed
- the air filter changed

Now only the low idle emissions seem to be the problem. All readings were fine as I left the garage last time but by the time I got them tested at the NCT centre just a couple of hours later, the high idle emissions and the lambda readings were OK but the low idle CO levels were up at a whopping 3.47 vol % again (the limit for my car is 0.5 %).

My mechanic doesn't know what to do. He rang the Renault guys in Dublin and they couldn't give him any pointers either. I rang an emissions specialist and he said he is having a Megane coming in sometime soon that seems to have exactly the same problem - it's been to various garages and all standard things have been looked at, still the emissions are fluctuating arbitrarily. He said he hadn't come across something like that before and he'll have to experiment.

Would anyone have any suggestions at all? I've only had the car for a few months and didn't see this coming as it is otherwise running perfectly!

Many thanks,
Squiggers