the flame rod circuit is from the flame rod to machine ground. the test setup goes from the flamerod wire to a good ground on the heater. if the flame goes out again, reverse the test connection. the flamerod should be positioned in the middle between the pot and the burner ring.
the way it operates is there's 120 volts on the flamerod in the absence of a flame. once there is a flame, the air around the flamerod is ionized and conducts, however at this point, the ac changes to dc and is applied to the flame detector circuit on the motherboard.