Why does my furnace turn on and then shut off after a few seconds?

This most often points to a fouled flame sensor, the metal rod that confirms a flame is present. When it cannot detect the flame, the control board shuts off the gas as a safety measure, so the furnace lights and then quits. Cleaning or replacing the sensor usually resolves it, though a dirty burner or a failing control board can cause the same symptom, so a technician confirms the cause before repairing.

Is it safe to keep running a furnace that keeps shutting itself off?

It is better to have it diagnosed before running it repeatedly. A furnace that keeps shutting down is usually responding to a real fault, such as restricted airflow, an overheating limit switch, or a flame sensor dropping the flame, and in some cases the lockout is protecting against an unsafe condition like a cracked heat exchanger. Repeatedly resetting it can mask a problem that needs attention, so a professional diagnosis is the safer path.

Should I repair or replace my furnace?

It depends on the furnace's age, the cost of the repair, and its safety. A unit more than about fifteen years old that needs a major component, or one with a cracked heat exchanger, is often better replaced, while a newer furnace with a single failed part is usually worth repairing. A technician can weigh the repair cost against the age and efficiency of the system and present both options so you can decide.

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