I have a Trane forced air furnace which recently has had periods when it blows unheated air. When I looked at the furnace during these periods it shows 2 flashes, which according to the instruction manual is an error code which means: System lockout, retries or recycles exceeded.

I don't understand what this means and what is wrong with the furnace. Why does it blow warm air sometimes and unheated air at other times?

Thanks for your help.