Lots and lots of folks ask this same question and it is. Help my furnace runs for 20 seconds and shuts down. What could be the problem. This is how it works. When you call for heat a small fan comes on and cause a draft over the burner chamber. Sort of like a nice way to add oxygen to the space shuttle for premium blastoff(well sorta) Anyway the brain gets continuity from the vacuum switch telling it to go ahead and bust a move. so it fires up the ignitor and then it energizes the gas operator solenoid. This open the gate inside the gas valve, raw gas travels out and ignites on the glowing red ignitor.But already the brain(control) is worried. Why? Because it hasn't heard back from the FLAME SENSOR. The FS sends a signal concurring their is indeed ignition and raw gas is not spewing out all nakely into your house without being burned!! Well what can be done? Sometimes sanding the contaminated flame sensor can make the furnace work again. Most of the time the flame sensor needs replacing. Where is it? Looking at the Brain their is usually an ORANGE wire. it leads to a ceramic base. On that piece of ceramic it the alleged FLAME SENSOR. Clean it or replace it and the furnace will work again 99% of the time.