This is the most popular question in heating today and one way that repairpeople make a living from a hopeless situtation. The sensor for the flame does not prove that there is a flame and stops the gas from flowing. Household cleaning fluids and other chemicals in the air coat the sensors and make them unresponsive, so the sensor must be cleaned by rubbing it with paper or fine steel wool (being careful to remove all strands so there is no short) or replacing the sensor. At times, the sensor must be replaced or the board fails. This must bring in several million dollars to the repair shops and perhaps the manufacturers per year.