2009 Vauxhall Insignia engine management light coming on

occasionally the management light comes on and when i plug in my code reader it informs me that air/fuel mixture is( Lean bank 1 ) I can delete this message and it can be a a few days later when the same fault is repeated.Can anyone advise on possible fault without having to get expensive diagnostic test carried out.Car is showing no adverse effects with management off or on,and i use the car daily