The furnace turns on, runs a few minutes, and shuts off — over and over. It is very often the filter.
A furnace's blower has to move a specific volume of air across the heat exchanger to keep it within its designed temperature range. A filter clogged with a season or two of dust restricts that airflow, the exchanger runs hotter than it should, and the high-limit switch — a safety device — trips to shut the burners down before things get worse.
The furnace fires, runs for a few minutes, and shuts off well before the house has warmed up, then starts again a short time later. It is not lighting and quitting like an ignition fault — it runs for a real stretch first, which is the tell that airflow, not ignition, is the likely cause.
The high-limit switch does not know why the exchanger is hot — it just measures temperature and reacts. Restricted airflow is one of the most common reasons the exchanger runs hotter than normal, and a dirty filter is one of the most common reasons airflow is restricted. It is a simple chain, and it is worth ruling out before anything more involved gets inspected.
A filter is the easiest thing to check, but it is not the only one. Closed or blocked supply registers, a blower wheel caked with dust, or ductwork that was undersized to begin with can restrict airflow the same way a dirty filter does, and any of them can produce the same short-cycling pattern.
If a new filter does not resolve short cycling, the airflow restriction is somewhere else in the system, or the problem is not airflow at all — a failing blower motor or a genuinely developing heat exchanger issue can produce a similar pattern and needs a proper inspection rather than another filter swap.
Checking the filter monthly during the heating season, and replacing it before it looks obviously dirty, prevents a meaningful share of the short-cycling calls furnace contractors get every winter.
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