Hi, I’m Vance and welcome to Repair and Replace. If your furnace starts, then stops, and doesthis several times, then it’s likely a safety switchhas tripped or has failed. In this episode, we’ll look at the causesof short cycling. We’ll briefly cover each repair, but if youneed more information then watch the videos linked in the description. To troubleshoot short cycling, we’re goingto look at a few specific scenarios, 1: the furnace shuts off before ignition,2: the furnace ignites but the flames won’t stay lit, and 3: the furnace ignites, blows warm air but shuts off after a minute.Lets begin. Modern furnaces have a set of safety switchesthat shut down the furnace when it detects an unsafe condition. This includes the pressure switch, flame sensor,flame roll out switch, and limit switch. If a switch is tripped, it will cause thefurnace to lockout. It will cycle on and attempt ignition several times before going into a hard lockoutfor up to an hour. Every time a lockout happens, the furnacewill use flashing lights as codes to identify the source of the shutdown. These codes are unique to each model, so checkyour furnace manual to see what each code means. Even without any furnace codes, you can stilltroubleshoot the problem by observing the furnace. Keep in mind, if you are going to troubleshootyour furnace, always be safe and turn off the power and gas to your furnace before doingany repairs.If the draft inducer fan runs, but the furnaceshuts off before igniting, it could be a problem with the pressure switch. When the thermostat calls for heat, the draftinducer pulls fresh air into the burner. This clears the burner of exhaust gases andallows for safe combustion When the pressure is low,the switch will stay open and will prevent the furnace from igniting. Try to identify anything that might causea change of pressure inside the heat exchanger. Check to see if there’s any obstructions inthe flue vent or in the inducer motor port.Next check to see if there are any blockagesor water in the pressure switch tubing. If the airflow is good, then the pressureswitch might be faulty. Alternatively, if the burners still don’t ignite,then the furnace ignitor might of failed. The ignitor can be tested for continuity with a multimeter to see if a continuouselectrical path is present. If there is no continuity,then replace the furnace ignitor. If the burners shut off shortly after igniting,then the flame sensor could be dirty or defective. When the airflow is good, the hot surfaceignitor will heat up, the gas valve will openand the burners will ignite. The flame sensor monitors the burners, and will shut the furnace off if there’s no flame present after 7 seconds.If the flame sensor is dirty, the furnacemight shut off even when there’s a flame. Remove the sensor and clean it with a softabrasive pad. Now if the burner still doesn’t stay on, thenreplace the flame sensor. After ignition, the blower fan will run andcirculate the warm air around your home. If the furnace ignites, the blower fan runs, but the furnace shuts offbefore the house warms up, then the filter could be dirty,or the limit switch may be faulty.The limit switch monitors the temperatureof the air passing over the heat exchanger and will shut off the furnace if the systemoverheats. Any restriction of airflow might cause theheat exchanger to overheat. This is why it’s essential to replace thefurnace filter every 3 – 6 months. If you’ve replaced the filter, and the furnace still continues to shut off, then check the limit switch. The limit switch can be tested for continuity. If the switch has continuity, the multimeterwill show zero ohms. Even if the limit switch has continuity, itmight still be faulty. As a limit switch fails, it can start shuttingthe furnace off at lower and lower temperatures.So if the limit switch has failedor is starting to fail, then it’s best to replace it. If you liked this and want to see more tutorialsand informational videos, then subscribe to our channel. And if you need help, you can call or visitan AMRE location to talk with ourknowledgeable staff. Thanks for watching..