Ok,the flame sensor is just a wire. It does not change resistance with heat. Also the flame sensor has only 1 connection and as you shoild know, a complete circuit needs 2. The circuit is completed THROUGH THE FLAME to the metal burner. And the current does not come from the controller. The flame actually generates the current. The controller senses therescense of thos small current. So the flame is a small battery where the neg terminal is the metal burner and the positive terminal is the probe in the flame. The current is very small and i would not try to measure it unless you have the right kind of meter and know what to do.

Also the flame sensor in NOT like the thermocoouples which have two wires and generate current inthe two wires when heated. The flame sensor is different.

The flowing hot gas from the burner tothe probe creates a small current.

I donT know how important it is for the probe itself to be clean, but the insulator that holds the probe needs to be clean or else the small current willleak directly back to the grounded metal ans not make it to the controller.



Mark