I have a a 12 yrs old Arcoaire RPJ 2 furnace. I cannot tell from the owners manual the exact location of the flame sensor. It is shown in the exploded parts diagram, but it is not obvious to me where it is. I cannot see it on the furnace. Is it necessary to remove something to expose the location ? I need to clean it to see if it cures the problem of the burners shutting off after about 20 seconds. I installed a new control module and the problem is still there.