While checking the operation of a furnace, the thermostat is set to its highest (or warmest) setting to make the furnace operate. 10 seconds after the furnace contacts close the burners ignite and cause a loud noise and flames shoot out of the front. Which of the following is the cause.

While checking the operation of a furnace, the thermostat is set to its highest (or warmest) setting to make the furnace operate. 10 seconds after the furnace contacts close the burners ignite and cause a loud noise and flames shoot out of the front. Which of the following is the cause.