We have a 5-6 year old Trane XR80 furnace. Intermittently it will exhibit the following behavior:

1. Thermostat calls for heat
   
2. Draft motor starts and runs for 15-20 sec.
   
3. "Click" is heard (presumably gas valve opening or ignitor)
   
4. Nothing happens (normally burners would light now)
   
5. Draft motor continues to run, then stops, starts again. On second (or rarely a third) attempt, burners do light and furnace runs normally.

When I had a clean and check done last fall, I relayed this problem to the tech and he confidently said it was likely a dirty flame sensor. He cleaned the sensor with fine sandpaper and polished with steel wool. Reinserted and the furnance ran flawlessly for a couple months before the behavior returned sporadically. Furnce was inspected and flame sensor cleaned again that winter and reduced incident rate of problem, yet problem still occurred not more than 4-5 hours after tech had cleaned flame sensor. Now it's happening again...this morning twice in a row when the thermostat called for heat.

I've read where flame sensor issues usually involve the burners lighting and shutting down after a few seconds. In my situation, the burners don't even light. There is a glow from within the unit and a NG smell, so I know the ignitor is presumably working and the unit is getting gas.

While I've never experienced a situation with this furnace where the burners failed to light enough times to lock it down, it's unnerving to hear the unit cycling on and off trying to light.

Is this really the result of a dirty flame sensor? Other than getting dirty, can flame sensors "go bad?" How long do they usually last? If this isn't a flame sensor issue, what might it be? My logic is pointing to the ignition since the burners don't even light off, but I'm no HVAC professional.