How old? Can you tell what the sequence of operations is, or which operation is making it fail?
Take the top cover off and set the thermostat to call for heat. Your sequence will be inducer motor for 10-20 sec. then through the sight glass you'll watch the ignitor glow then you'll hear the gas valve open and you should have flame. Wait 30 sec to 1 min for the blower to kick on. Figure out what it is and is not doing.
If it kicks on and shuts back off, you might have a dirty flame sensor.