[Spokaneman]

My four year old Payne 90% efficient furnace fails to retain its flame and shuts down. The LED code says that the problem is in the Flame Proving circuit. The recommendation is for cleaning or replacing the "probe". I have done both with no success. This seems to be a common problem with Payne/Carrier units.

Can you explain how the flame proving circuit works? They may help me determine the next troubleshooting step.

[ballengerb1]

It is much like an electronic version of a thermocouple. Both are designed to close of the fuel if there is no flame present.

[Spokaneman]

It is much like an electronic version of a thermocouple. Both are designed to close of the fuel if there is no flame present.

I understand that much, and as I understand it, from reading the literature, the flame proving sensor completes a path to ground using the ion path in the hot flame as the path to ground as opposed to a typical bimetallic thermocouple creating a current to be sensed by the unit. The sensing circuit produces a current to the probe, and determines if that current is able to go to ground through the ionized gas, if yes, there is a flame = good, if not = bad = shutdown. (Seems like a rube goldberg way to do it, but so be it).

The literature says to clean the probe, but says nothing about cleaning the fire box or flame orifice. I would assume that if the current path was sensitive to oxidation on the probe, it would similarly be sensitive to oxidation where ever that current leaves the ion path and re-enteres the conductive elements of the furnace chassis.

What I am trying to figure out is whether that alternate location is specific location in the firebox, or just a general path to whatever conductive surface the current can find.

I assume if I have replaced the probe and cleaned the firebox, the next likely source of the problem would be the circuit board. Can those be tested, or is it just replace and hope?