Re: Faulty lambda sensor.
I have a Blue 2.2 Brera which I've had from new June 06 and last year (Feb 08) my 'Have Engine Checked' light came on. My dealer told me not to worry and that it would go off soon. It didn't. After several attempts at various 'faulty' sensors they couldn't make the light stay off. Eventually the dealership passed it over to ARUK. Their mechanic said the 'cam chain had stretched'. To fix this they fitted a new cam chain which meant a new engine being sent from Italy, stripped down and fitted to my engine. It would have been simpler to change engines. The light has stayed out and the performance seems to be much improved.
I hope your problem isn't the same as my Brera was at the Dealership for 10 weeks before it was fixed. In that time I drove a 147, a Golf GTI and an Astra, all at Alfas expense. The hire company who supplied the cars kept wanting to change the car due to the timeframe! It was really embaressing having to explain it to friends and business colleagues everytime. Didn't do anything for Alfas reputation either. I still love my Brera though. Good luck.