I Have a mid 1990's York furnace. After turning it on, the gas flame ignites, but 3 or 4 seconds later shuts off. Can you help?

Very likely the flame rod needs cleaning. It is a small metal rod with a porcelain insulator at the base. Rod sticks into the flame. Locate, remove, clean with a Scotch Brite pad or very fine sandpaper, & re-install.