I have an Ariston micro genus combi boiler G.C.N. 47-116-14 . When I turn on the water tap the boiler does not light and the yellow l.e.d. just above the on button lights up, the book calls it the "fume sensor" l.e.d. I cannot find a sensor in the machine. The fan is running ok. Anyone know what causes the fume sensor l.e.d. to light and stop the boiler lighting. many thanks for any replies.