Furnace Repair, Diagnosed Before Anything Is Replaced

This is one of the most common heating complaints, and it often traces to the flame sensor. If the sensor cannot confirm a flame — usually because of a thin layer of residue — the control closes the gas valve as a safety measure and retries. Other causes include a weak igniter, a dirty burner or a venting restriction, which is why the full sequence is checked rather than assuming the answer.

Most modern furnaces have an LED on the control board that flashes a diagnostic pattern, with a legend printed inside the access panel. The code narrows the search — a pressure switch fault, a flame failure, an open limit switch — but it describes a symptom rather than the root cause. A code pointing to the pressure switch, for example, may actually mean a blocked vent or a failing inducer motor.

Treat either as a safety matter first. Leave the home, avoid operating switches or appliances, and contact your gas utility or 911 from outside, following their instructions. Once the home has been cleared, a furnace inspection can look at the heat exchanger, venting and combustion to help identify the source. Carbon monoxide alarms on every level, and near sleeping areas, are widely recommended.

It can. A heavily loaded filter restricts airflow across the heat exchanger, which then runs hotter than intended. The high-limit switch senses this and shuts the burners off to protect the furnace, so the system cycles on and off and the house never quite warms. Replacing the filter is the first thing to check, but repeated limit trips are worth a proper look at airflow and ductwork.