When this happens, the FIRST thing you do is verify the LP system is ON and your range burners(all 3 or 4) are ON and have good flames. Then start the furnace, Go outside to the outside vent. When you hear the Ignition start to click, you should smell LP out of the exhaust vent IF the system is allowing LP to flow. IF you smell LP, then your burner is probably dirty or has insect nests. Also, possibly the Spark electrode is not gapped correctly for a hot spark. IF you smell NO LP, then you will need to pull the furnace to determine if it is a gas valve or a electrical issue at the valve or the LP burner orifice is clogged. Doug