fan is idle when it is supposed to be slow running?

The boiler(Vokera 80sp) fires up from cold when main power is switched on, it stops about 10 mins and cuts off and won't fire again, unless swith off main power and wait for few minutes and switch on. I found out the problem is when it cuts off , the fan is idle( it should slow run when boilt is not firing up). The slow speed resistor is ok, measured as 573 ohms. Fan is about 4 year old, runing pretty good. Flow switch is performing well. Which components are faulty to cause this problem? A faulty PCB?