I have a Trane gas furnace w/no standing pilot it's electronic. Anyway when the t-stat calls for heat it will fire up and the shut of within 5 secs. It will do this approx. 4 times before it will eventually lock itself out and the blower will continue to run not ever reaching setpoint. I know that a real dirty filter could cause this but is there a safety that could cause this because my filter is OK? ( If a safety is open wouldn't it not let it fire at all?)

Any help would be appreciated!

Remove the top access panel of the furnace. Cycle the furnace and watch through the inspection window at the burners. You should see an orange glow and ten the burners should ignite and the glow will turn blue. The flame should spread to all burners very quickly. If the flame takes more than 2 seconds, it will go out. If they are traveling fast but still going out, remove the burner cover and with a small brush, scrub the small probe found all the way to the left. This is your flame sensor. Prior to this, check the wire from the sensor down to the circuit board. I think it is a white wire.