I have had the emissions warning lamp come on a few times recently. I have taken the car back and the fault shows LEAN FUEL MIXTURE. 2007 2.0 Turbo S3. I took the garages advice and filled the tank with super plus unleaded, 99 octane rating. After half a tank the emissions light is on again. The car feels flat, no power at all when light is on. The light is on constant. I can rest the light by disconnecting the battery for a few seconds and i get full power back. Getting fed up now, light is back on after numerous tanks of 99 octane fuel. Any ideas what the fault could be. I am hoping its a sensor before or after the Cat. Could it be anything more serious.