The oven's fan that blows thru the front (not convection fan) sometimes stops working if oven is heated and then turned off and back on again. The last time I used it after the oven heated up and the preheat light went off the on light stayed on but the preheat never came back on and the oven stopped heating. After it was off for a while I turned it on and it is initially working again. Would this be a board or a thermostat?