janitrol furnace won't stay running

I have a janitrol furnace the other day it was hot out so i thought i would try my central air it worked fine, well now it is in the 30s and when i turn my furnace on the igniter glows and the burner turns on and burns for 5-10 seconds then it goes out and does the same thing in a couple of minutes. i had a new igniter i put on and it does the same thing, i was thinking maybe it might be the flame sensor but i don't know how to test it thank you for any other help you can supply.