My furnace was working great. Then sometime last night it quit putting out heat. The blower was running and works fine but no heat. Yes, I have propane in it. Please help we're getting cold! It is a 1976.

Number 1 cause of this if it had been running OK for a while is that your battery voltage has dropped to a point where the sail switch (airflow detector) opens up. Then the ignitor/gas shuts down for safety because it thinks the blower is not turning. Believe it or not, the fan continues to run forever (til the battery fails) and that is normal operation. Charge the batteries and try again. If still have a problem, check the ignitor for sparking, gas jet and thermocouple for a true failure...but if it had just been working, no mud-daubers should have gotten in there yet and alignment of thermocouple should not have changed. Battery voltage sure would have over the evening if dry-camping or if your converter had not been switched on. Good luck!