The ignition control board is the furnace's computer. It runs the ignition sequence in the right order — start the inducer, confirm the pressure switch, energize the igniter, open the gas valve, prove flame at the flame sensor, then start the blower — and it monitors the safety circuit the whole time. A blinking LED on the board flashes diagnostic fault codes that tell a technician where in that sequence something failed.
Because the board touches everything, a failed one produces varied symptoms: a completely dead furnace, components that fire out of order or not at all, erratic operation, or no response to the thermostat. Boards die from power surges, ordinary age, moisture (a real risk on condensing furnaces with a leaking condensate line), or a shorted component; sometimes you can see a scorched trace or smell burnt electronics.
A replacement board typically runs $300 to $1,200 installed — the part is $50 to $300 or more for advanced high-efficiency models, plus diagnostic and labor time. The board is also the most over-diagnosed part in the furnace: its symptoms overlap with a bad igniter, flame sensor, or thermostat, so it's an easy thing to blame. The diagnostic discipline that protects you is reading the board's own fault code and testing the cheaper components in the sequence first. Be wary of a board (or whole-furnace) quote offered before anyone has pulled the flash code and ruled out the $30 parts.