You do not say the make & model of your boiler...
If the pilot lights but the main burner does not then the fault may be...
1) Weak Pilot Flame failing to generate enough heat for the heat sensor
2) Heat sensor defective
3) Heat sensor sending correct signal to PCB but PCB not acting on it
4) PCB asking main gas valve to open but valve failing to operate.
I am not qualified or experienced to offer this opinion.
Good luck...
Peccavi, August 2010
