My furnace- a Trane XL80 lights for maybe 10 seconds then shuts off. It will do this for several cycles, then quit. If I turn off the thermostat and turn it on again, the sequence will repeat. I have looked into the problem and it seems that the exhaust fan (located at the bottom of the flue) isn't starting up is there a test or a switch or something to check whether the motor is good or maybe a sensor or capacitor or something else is kaput?

It was the flame sensor- got it fixed w about fifty cents worth of steel wool