Your furnace relies on various sensors just like your vehicle, but it's often more than a minor inconvenience when one fails. Your furnace's sensors help it to operate safely and reliably, so a failed sensor will typically cause your furnace to stop working altogether. Fortunately, furnace sensor failures do have one thing in common with their automotive counterparts: they're usually relatively inexpensive to repair. If your furnace has stopped functioning or it's behaving erratically, it may be due to one of these three sensors.
1. Flame Sensor
The name tells you all that you need to know about this critical component. Gas furnaces work by burning a continuous stream of natural gas. Once the furnace ignites, the flame will keep going as long as fuel is available. If the burner stops or fails to ignite, then the furnace must shut off its fuel supply to avoid pumping gas into your home indefinitely.
The purpose of the flame sensor is to confirm ignition. If the sensor indicates that the flame isn't burning, your furnace will stop supply gas to the burner. Since these components are critical for your furnace's safe operation, a failure will prevent your furnace from igniting altogether. Faulty flame sensors can also cause your furnace to turn on for only a brief moment before shutting off.
2. Draft Pressure Switch
You can find draft pressure switches on modern and high-efficiency furnaces. These furnaces utilize a secondary blower (a draft inducer) to drive fresh air and exhaust gases through the unit. The draft pressure switch tests for negative pressure inside the furnace to confirm that exhaust gases are not building up or leaking into your home.
As with the flame sensor, a faulty draft pressure switch will fail in a safe position. This behavior means that if your draft pressure switch goes bad, your furnace will fail to turn on. If you hear the draft blower turn on and then turn off shortly afterward, a faulty draft pressure switch may be to blame.
3. Limit Switch
Your furnace's limit switch works with a temperature sensor to control when your furnace should turn on or off. The limit switch is responsible for engaging the blower motor once the burner begins to warm the air, but it also shuts the furnace down if it becomes too hot. Overheating can damage your furnace's heat exchanger, so this high-limit function is crucial to keeping your furnace safe and reliable.
When your limit switch fails in a tripped (high limit) position, it will stop your furnace from turning on. In some cases, this may not be the limit switch's fault, however. If your furnace overheats often enough to trigger the sensor a pre-determined number of times, your furnace will go into a safe mode until you address the underlying problem. Alternatively, a heating system service can provide additional information.