In my old house I had two of those furnaces. (Main house and granny suite)
The flame sensors were a big problem.
I was cleaning them constantly. It finally got to a point where I just bought new ones every year and changed them.
A gas tech friend of mine says that it was because I was drawing combustion air from inside the home.
Anyway, resetting the power to the furnace usually works.
I notice when i clean the flame sensors i can move it up and down with it still being screw on the frame. Could that be a problem, i notice the flame sensor not heating up to transfer the flames to the valves. I believe i have reset the power, but turning the switch on and off. Is that correct?