I would say on a 15 he,the heat ex is probably the same as a 133,barcelona etc,one of the main problems with these models is the failure of combustion door seals/sump seals/flue seals which generally manifest as a fan sensor o/heat fault.As a BG eng,the LAST thing I would look at for this fault...
more than likely to be the h/w diaphragm,if the hw indicator light has not illuminated it means the micro switch has not made,unfortunately not the easiest of things to change :cry: