a method of determining presence of a flame in a furnace of a heating , ventilation , and air conditioning system . the method comprises determining , using a controller , whether a processor signal is active , responsive to a determination that the processor signal is active , determining , using the controller prior to assertion of a flame - test input control signal , an output state of a first comparator , responsive to a determination that the output state of the first comparator is high , determining , using the controller prior to assertion of the flame - test input control signal , an output state of a second comparator , and responsive to a determination that the output state of the second comparator is low , transmitting , using the controller , a notification that a flame is present .