I have a 2007 Bryant Plus 90t Furnace and since it was installed 1n 2007 the Flame Sensor had to be cleaned 3 times, what's the problem why does the flam sensor get corroded and the furnace won't light.

Whatever is in the air in your home is the usual cause of the flame sensor getting a coating on it. It could be many items from laundry supplies to hair spray.

Observe the combustion vent terminations while the unit is running. Are exhaust vapors being drawn into the air intake? If so, the terminations may have to be re-worked.