after driving the s4 continually for 6hrs today the engine managment light apeared on the dash, no symtoms at all just the light. re started engine - still no good - drove all the way home - light still on, anyway was going to book it in to beechwood audi tommorow when i decided to disconnect the battery forr 10mins first. result - light gone! all reset!
worth trying this first before you hand over an hours labour at the dealers!