24th July test: CO emissions at high idle: 0.35% (limit 0.3%), lambda 0.99 (range allowed 0.97 to 1.03)

22nd August: bought new cat, had fitted

23rd August test: CO emissions 0.41%, lambda now 1.28. In previous attempts CO was at about 0.6 but the lambda was OK, but suddenly the lambda went out of range and the CO fell a bit.

Makes no sense to me.