Thanks nocturnblue for taking the time to help find a solution, I would think if there was an exhaust leak the garage who is working with my car at the moment I would like to think had checked for leaks as he has had the car up on his lift. I do have a Bosal cat which I had fitted but the readings were the same, turns out the lambda was not working, the one on now is supposedly working but still slightly high co, I fitted a new coolant sensor for the ECU but I think it was no good as the radiator fan came on immediately. He thinks he can do something with the feeling using his Dyno/ rolling road equipment but this I find hard to believe as it cannot read my ECU. I am waiting for a Pscan so will then take the car back and hook it up , one garage I phoned said my belt could of slipped one tooth, not checked this as the car runs very well. I like my MG but I seem to have had every problem an MGF can get over a period of time, this has been the toughest one to sort out, I have renewed so much mechanically , the car can pass an mot no bother except for the emissions.The lambda seems to have failed the first fast idle test but passed the second one, so the MOT says. Hmmm.
I'm no expert on MOT tests but both fast RPM idle tests look to be conducted exactly the same. Maybe things were hotter the second time around...
As I understand it a lambda responds to the O2 concentration in the exhaust and goes high voltage (towards 1V) when the exhaust is fuel rich as there is little O2 available then - they act like little max. ~1V batteries which work on the difference in O2 between their exhaust gas end and their other end which gets fresh air.
A bit of digging around lead me to an HonestJohn article which says that the MOT testers don't actually measure the lambda sensor output voltage of the car under test, they calculate what it would be (in theory) from the measurements from their exhaust gas analyser.
This makes me wonder whether the lambda sensor voltage MOT failure isn't just an effect of the high CO reading, rather than the cause? In other words, fix the high CO reading and the theoretical MOT lambda voltage will apparently come within limits again? Thinking about it, if they calculate a theoretical lambda voltage using the emissions test data, it kind of is assuming the lambda sensor is the sole factor in emissions whereas as we all know the ECU relies on engine sensor data as well as the lambda to gauge stoichiometric fuel delivery conditions.
After all, the OP has already changed the cat, what are the chances it's still the cat causing the high CO reading?
Still sounds like a sensor problem to me.
By the way, check for exhaust leaks around the new cat, apparently that can alter emissions readings too. Here's an excerpt from the MOT tester forums regarding someone who had failed on the lambda but passed after sealing the cat joints better:
'i think im the tester who tested your mr2 on wednsday test and advised that the catalyst joint had a minor leak of exhaust gasses. this usually causes a high lambda reading and certainly wont help it getting through the emissions. '
So, case in point, there was nothing wrong with the lambda in that case, an exhaust leak made it look like there was.
cheers and thanks Dean.