I have a bryant furnace about 7yrs old for the last 5yrs I have had to have service on it the company says it needed cleaning it calls for heat it cycles the glow bar heats up flames ignite then 3 saconds later goes out I turn the power switch to off for twenty seconds then it will work for awhile sometimes it takes acouple of times to get it working .

The flame sensor most likely has built up soot on it. When not working properly, it doesn't detect the heat given off by the flame so it shuts the gas off. This is what the service company does... yearly? First turn off power to the furnace. Find the flame sensor inside the furnace. It's a long wire attached to a metal rod that usually has a white ceramic base. Sand the flame sensor gently with a clean emery cloth or steel wool. This will remove built up soot preventing the flame sensor from working properly. This can sometimes be done without removing it from the unit.