Just as Kirk has stated that after a certain number of tries (depending on the DSI board) up to three if it fails to ignite it goes into lock out and on the older (prior to 2002) the fan continues to run. The newer models have been redesigned so the fan will stop also. The older furnaces can be upgraded with a new version DSI board and then the fan will stop in the event the furnaces fails to lite. I would highly recommend the Dinosaur brand Fan 50 plus DSI boards. They have a three year warranty and are a three try board. You will need to have someone that is knowlegable to change the fan wiring to incorporate the new version board.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7