Purchased a siemens fridge freezer in 2008, worked fine, then last year the fridge stopped working, engineer came along,fitted new fan and plate!worked fine for a week, he returned and fitted all new parts. after a year the problem has cropped up again, have defrosted it and it seems to be working, but for how long!it is still under warranty and i would like to keep it as it is really good when working. it does state in the manual that it is A+ rated, is frost free and does not need to be defrosted! please help