A control for controlling the operation of a fuel-fired heating appliance is provided that include a sensor capable of sensing the presence of carbon monoxide gas and providing an output indicative of the level of carbon monoxide gas. The control further includes a microprocessor that periodically reads the CO sensor output and stores at least one sensor output value in a memory, wherein the microprocessor responsively discontinues the operation of the appliance when the microprocessor detects a sensor output that has increased by more than a predetermined amount over at least one previously stored sensor output value. In one embodiment, the control is preferably configured to periodically read the sensor output value, and responsive discontinue operation of an appliance upon sensing a predetermined increase of at least a 50 percent increase over the previously monitored sensor output value over a period of about 15 to 90 minutes.