You need to find the troubleshooting part of the manual. On closed combustion systems, there are numerous interlocks. The system has a vacuum switch that verifies there isn't a leak or break in the exhaust lines. Course, the logic board could be shot, too. It could be something as simple as the vent being obstructed, or a crack or loose joint in one of the vent lines, among many other things. Often, the light will flash a code or something.