Modern gas- or oil-fired furnaces have several safety sensors that check on furnace operation. The furnace controller monitors these sensors constantly. If one of the sensors reports an unsafe condition, the furnace controller shuts off fuel and power to prevent unsafe operation. This condition is known as lock out. Your furnace normally will remain in the locked-out state until you manually reset it. Most furnace lock outs are caused by problems with igniters, flame sensors and limit switches.
Common causes of furnace lock outs are a defective igniter or defective flame sensor. When the furnace attempts to start, sensors check for igniter action and for the presence of flame. If the igniter doesn't activate within a few seconds, the furnace controller stops the ignition sequence. In most furnace models, the controller will retry the ignition sequence twice more before going into lock out.
If the igniter does activate but the flame sensor doesn't report the presence of flame within a few seconds, the furnace stops the ignition sequence. It will try twice more for ignition but if the flame sensor doesn't report flame, the furnace goes into lock out. Gas furnaces with pilot lights will go into lock out if the pilot light goes out or if the pilot light fails to light up the main burner.
Furnaces also include limit switches that monitor fuel pressure and the internal temperatures of the heat exchanger. If fuel pressure is too low, or if the temperature in the heat exchanger is too high, the switch closes. This tells the furnace to shut down and go into lock out mode. Sometimes a dirty air filter can reduce air flow through the furnace to a point where the heat exchanger limit switch trips. Many furnaces with electronic controls will have status code lights that can tell you what caused the furnace to lock out.
Once you have identified and corrected the cause of your furnace lock out, you must manually reset the furnace. Most gas furnaces can be reset by shutting off the electric power, waiting 20 seconds and turning the power back on. Some furnace controllers will unlock after an hour or two and will try to operate again. Most oil-burning furnaces have a reset button on the burner that you press to restart the furnace. If your gas furnace has a pilot light, you must relight it according to the manufacturer's directions. This should unlock the furnace.