I have a trane single packaged gas/electric 2 1/2 ton. The gas will light but after about 5 seconds goes off. It does this three times then the system goes into system lockout, no heat. You can reset power and it will start the whole process over. I have changed the ignition plug, flame detector and gas valve to NO avail!

Is it new enough to have a light that flashes trouble codes? There is a sensor that will shut shut the gas off if the blower doesn't come on before it gets too hot. The sensor could be bad and be shutting the gas off before it reaches the temperature where is should. Should be a little doohicky near the burner, but possibly on the air side of the heat exchanger. It will have a couple of wires running back to the control board.

michael bennett agrees: Thanks for the info! The circuit board does have an LED. It keeps giving me a system lockout(no flame). I have checked the schematic and there is a thermocouple, but I have checked that and there is good continuity? Any more help? Thanks!!

The thermocouple could still be giving the wrong signal even if it is continuous. Could be the circuit board too. $$$$$$$

Thanks again for your help!!