If your previously reliable furnace has started to flicker or shut down when it's running, that's a cause for concern and frustration. When it comes to narrowing down the source of the inconsistency, there are a few things that you can check before calling your furnace repair technician. Here are a couple of the most common reasons for a consistency problem with your furnace.

Flame Sensor Issues

If your furnace fires up right away with a consistent flame and then just shuts down after about thirty seconds, that may be a result of problems with the flame sensor. The flame sensor is a safety mechanism that detects the presence of the pilot flame to ensure that your furnace is burning the fuel that's being supplied. 

When the sensor is dirty or malfunctioning, it won't necessarily recognize that the pilot light is burning. As a result, it shuts down the fuel supply. This is evidenced by the sudden cutoff of the burner on the furnace. If it's shutting down quickly instead of sputtering, try cleaning the flame sensor first. Shut down the furnace and remove the cover from the burner panel. Use a damp, clean rag to remove any soot, smoke residue, or other debris from the sensor. Then, hit the furnace reset and try to fire it up again.

Fuel Supply Interruptions

If, instead of shutting down suddenly, your furnace flickers and sputters, that may be a result of inconsistent fuel supply. This can be caused by a variety of things. Most frequently, the fuel nozzle is often clogged with debris from the tank. If you've recently let your fuel tank run low or you've run the furnace while you were receiving an oil delivery, there may be something clogging the fuel nozzle and interrupting the fuel delivery. You can clean the nozzle out gently with a rag, but make sure the furnace is completely shut down first.

When the problem persists after cleaning the nozzle, you likely have air in the lines. If you know how to bleed your furnace, you can bleed it a couple of times to see if that fixes the problem. If it doesn't, you'll need to reach out to a furnace repair technician.

Persistent air in the fuel line can be an indication of a crack in the line somewhere, which means you may have fuel leaking and contaminating the property somewhere. It can also be an indication that one of the fittings on the lines is loose and allowing air to seep through. You'll need a technician to troubleshoot the problem and figure out what has happened.