A blower motor failure is detected by a sensor/switch that opens at a first preset temperature level and automatically resets for the next cycle. The occurrences of such openings per thermostat cycle are counted and when they reach a predetermined number, the microprocessor causes the system to lockout from further operation, thereby preventing the temperatures from reaching excessive levels. In order that the same system may be used in a horizontal furnace installation where the first sensor is less responsive because of its location, a second sensor, of the manually resettable type and having a preset temperature level for opening which is higher than said first temperature limit, is employed to thereby lockout the system.