The sequence is as follows...
Call for Heat (Thermostat + Programmer)
Fan Runs
Pressure Switch detects fan running - tells PCB
PCB opens pilot gas valve and makes spark
Pilot lights (automatically)
PCB senses heat from pilot light and opens main gas valve
Main Burner lights...
There is a good fault-finding chart in the back of the installation & servicing instruction booklet if you still have it.
Peccavi
December 2008