I have a Trane XR80 that was installed new in the summer of 2004. Tonight it appears to not be functioning correctly for the first time. The error light is blinking twice, I have replaced the air filter, made sure the pilot is on and the gas valve is open. The unit appears to turn on and ignite the burner for about 3 to 5 seconds and then shuts off.

Could be the Ignitor. I would suggest power cycling the system twice within 30 secs. I would also suggest calling and getting a service quote from a trained HVAC professional. If it is the ignitor? Should be fairly quick fix. But a fix that a professional should handle when working with gas units.