As Aidan has mentioned, there are revised limits on some of the N engines because of GDI strategy, this shows as very high Lambda at fast idle allthough the CO will be very low (bit difficult to be rich and lean at the same time!). If you have high CO then as well as the basics, these engines suffer from, plugs/coils and especially injector problems.
If the dealer supplied a good Exhaust gas test then they probably cleared all adaptions and tested it straight after!.