gas heater trouble
It sounds like you have a new enough furnace to have a flame sensor. The flame sensor is a metal rod in front of the burner that is opposite of the ignition source, possible ignition sources are hot surface, direct spark, intermittent pilot. If the unit uses a pilot the flame sensor could be located in the pilot burner. Look for a rod that is about 1/8"-3/16", it should have a single wire coming off from it. The wire could be yellow or white and some manufacturers use a orange one. Once you verify what you are dealing with, unplug or shut power off to furnace, remove the rod from the furnace, use a wire brush and clean very well. ( use wire brush only, DO NOT USE SANDPAPER OR SANDCLOTH ). Reinstall the flame sensor, turn power back on to furnace, unit should operate properly. If it does not stop the problem it could be ignition system is failing. Not sure what type or model furnace you are working with, but if this does not work if you can give me the information, I can walk you through it.