The error code tells you what sensor has tripped. It does not tell you why the fault. Do not blame the code sensor light unless you are positive it is the sensor that has failed.
Thermostat malfunctions are most common heating system failures so check the switches, settings and loose or broken connections. Other reasons are tripped circuit breaker, blown fuse, combustion furnace pilot light.
The furnace diagnostic code lights can be seen through the furnace window or when the front panel is removed.
Different furnaces have different patterns. For example, maybe the light codes are 2 digits – the number of short flashes is 1st digit of the two digit fault code. The number of long flashes of the LED is the second digit of the fault code. A steady burning LED light means normal furnace operation.
Or it means something if the blinking is slow or fast. Like the green light is blinking slowly means the furnace is on but there is no request for heat. A continuous light means that the circuit control board needs replacement, but completely off means the furnace is either off or not receiving electrical power.
If a code refers to gas, ignition failure, or flame proving failure, the problem is the burner assembly area, like the igniter or flame sensor is either bad or dirty.
An oil furnace not coming on may show a red light lit on the burner unit. When on, this red lockout light indicates you are shutdown due to a problem or fault for safety.
A furnace blower motor that stops pushing any air is broken. If there is a steady flow of cold air, the blower is good, but your heat exchanger or burner is not, and inspect for cracks and leaks.
Why does my furnace turn on and off? It could be the flame sensor is dirty.
But the most common cause for your furnace to keep turning on and off is the High-Limit Switch, which turns the unit off if the temperature inside gets too high, as a precaution. This is most common cause of short cycling. A dirty air filter restricts air from getting into your furnace. Your furnace’s heat exchanger needs that air blowing over it or it overheats, causing the furnace to shut down. So, change it!