I have a Trane XL 80 two stage furnace which has been acting strangely the last couple of weeks.
When the heater is started, the igniter comes on and the when the gas starts to flow the burner starts on. However within a couple of seconds the burner turns off. The burner restarts after a minute or so and again goes through the same set of steps. After three retries it stops and the control unit shows 2 flashes which is code for "Flame not available, check control polarity"

At certain times the burner starts of correctly and continues. It continues to work till at a later time the heater is stopped (after temperature setting is reached) and then has to restart.

I don't believe the controls are bad or the polarity is messed up (else it would have never worked). My guess is that the gas pressure is not sufficient or something else is causing the flame check control to report an error. Any ideas?

Try cleaning the flame senser,, turn off power for few seconds and back on to reset system

I agree with wmproop

Thanks for the tips. I will try that and let you know the results.
What's the best way to clean the sensor - green scrub pad or sand paper. I noticed that it can be removed if the small screw is taken out. Is that recommended or in place cleaning is fine?


Thanks for all the help.

Sand paper works fine just take care as to not damage the ignitor. In some cases removal and then cleaning might be beat.

Thanks
In my furnace I have 3 burners. The igniter is near burner 3 and the flame sensor near burner one.
So I guess I wouldn't cause any issue inadvertently.


Thanks

I cleaned up the sensor and started the heater. Now I have a new problem. The burner starts fine and continues for about 5- 10 minutes and then goes off even though the thermostat temparature has not been achieved. After that the system tries to start the burner but it does not start.
When I tried to reset the system and restart, I noticed that the igniter comes on when the heat is called. But the burner does not start. That indicates that the Gas was not turned on. The system tries to start the burners three times - each time with the igniter coming on and then going away after a couple of seconds.
It seems like either the control is not issuing the call to the gas switches to turn on or gas switches has gone bad.

Any ideas how to address this problem.

Thanks