Those two wires you tested on chimney......they go to the stack temp sensor.
Follow those wires back from chimney to the control module. Where they plug in is the circuit board/power module.
Should be under a cover. Same place that the end of 110V AC power cord goes.
Should have a 5A glass fuse for 110V AC circuit (electric heater) and a 3A auto blade type fuse for the 12V DC control circuit on the board.
UNLESS: You have a small manual fridge with controls on lower front (or top) then there is no circuit board.
