On start-up the fan starts-up and shortly after there is a click as I presume a solenoid Closes but there- after nothing. No pilot light is evident and of course full ignition is not initiated.
Gas is switched"on"
I did turn the "Control Knob" to a higher setting recently which may have induced an overtemp cutout situation.
Overheat stat may need a manual reset.
These are notorious for pcb's part number 407676 or 407677 Im not 100% sure but I think its the latter of those for your boiler
