My car went for MOT this morning. Watching the machine, I saw that CO was above 0.2% (about .25%) at fast idle. He revved the car up a bit and the CO came down but lambda went up out of range. When he dropped the revs, lambda and CO were good for 2 or 3 seconds and then CO went up to 0.25% again.

Now the car is 16 years old so doesn't have to pass the BET so got a tick on the MOT tester's check sheet but I'm interested why lambda should be too high when revved and CO too high at fast idle. What do you think?

When you say lambda went up when revved, do you mean while the revs were increasing, or whilst held at a steady 2-3000 rpm?

when revved at 4000-5000 (red line at 8500) lambda went up to about 1.1 as far as I could read. when held in the 2000-3000 band, lambda was in the green but CO went up above 0.2.