The furnace lights, runs for a few seconds, and quits. Here is what is actually happening.
A modern gas furnace will not run blind. Before it opens the gas valve, it proves the inducer motor is pulling enough draft, and after the burners light, it proves a flame actually exists before it keeps the gas flowing. Every fault in this part of the sequence is the furnace refusing to do something unsafe, not the furnace being broken in some dramatic sense.
Before gas is ever released, the inducer motor has to create enough vacuum to pull a pressure switch closed, which tells the control board the venting is clear enough to burn safely. A weak inducer, a blocked vent, or condensate sitting in a line can all keep that switch from closing, and the furnace will sit there trying — or lock out — without ever opening the gas valve.
Most furnaces built in the last couple of decades use a hot surface igniter — a ceramic element that glows to light the gas rather than a standing pilot. It is a wear part, and it can fail gradually, glowing dimmer over time, or all at once.
Once lit, a metal rod called the flame sensor confirms the flame is real by reading a tiny electrical signal through it. Over a season or two the rod picks up a layer of oxide from normal combustion, which insulates it. It can still detect the flame just long enough to let the furnace start, then lose the signal a few seconds in — so the furnace shuts the gas off as designed. That is the classic light-then-quit pattern, and it is usually a sensor that needs cleaning or replacing, not a bigger problem.
A control board that fails to confirm ignition a set number of times in a row will lock itself out rather than keep trying indefinitely — another safety measure. Clearing the lockout gets you one more attempt; it does not fix whatever caused the failures.
If the furnace lights and then dies within seconds, repeatedly, that pattern points fairly specifically at the flame sensor before anything else. It is worth mentioning that exact symptom when you call — it narrows down the visit before anyone arrives.
Describe what the furnace is doing and we'll match you with a licensed contractor who can act on it.
Get a Furnace Contractor OutA short description is enough — we'll follow up to confirm the details and get someone scheduled.