If heat is being called for the fan will run.
The fan runs and a pressure switch detects that it's working properly and then the light up process can begin.
The fan may be performing poorly
The pressure switch or a tube to it may be defective
The case seal may be defective
There may be a broken electrical connection to the switch
The control board may be failing to recognise the signal from the pressure switch or failing to activate the next stage in the start up cycle
Peccavi, January 2009
