The circuit board is the brains of the furnace and has many functions. It receives information from the sail switch to start the burner assembly and receives information from the high limit switch which can shut the unit down. If your troubleshooting steps have taken you to the circuit board, there is very little diagnostics that can be performed. Replacing the board is less expensive than taking it out and getting it checked.
Heating System Session 6: Circuit Board Replacement
