The sight of a steady pilot flame while the house remains cold indicates the furnace has gas supply and a basic ignition source, yet a safety or control mechanism is preventing the main burner from firing up. Understanding the sequence of operations inside a gas furnace allows for targeted, safe troubleshooting before calling a professional technician. This guide provides a practical, step-by-step approach to diagnosing the most frequent causes of this common heating failure. We will focus only on checks that can be performed safely and without specialized tools.
Check the Thermostat and Power
Before examining the furnace, verify the simplest external controls, as they are often the source of the problem. Confirm the thermostat is set to “Heat” mode, not “Cool” or “Fan Only.” Raise the temperature setting at least five degrees above the current room temperature to ensure a clear demand signal is sent to the heating unit. If the thermostat is digital, check and replace internal batteries if necessary, as a low charge can prevent the unit from communicating the heat request.
Locate the furnace’s dedicated power switch, typically a standard light switch near the unit, and ensure it is in the “On” position. This supplies electrical current to the control board and blower motor. Finally, check the main electrical panel to ensure the circuit breaker dedicated to the furnace has not tripped. A tripped breaker can be reset by flipping it completely off and then back on.
The Flame Sensor or Thermocouple Problem
After confirming external power and control signals, focus on the safety mechanism that proves the pilot flame is present. Modern furnaces use a flame sensor, while older units use a thermocouple. Both devices operate on the principle of flame rectification, generating a small electrical current or voltage when heated by the pilot flame. This signal is sent back to the main control board, acting as a safety interlock that permits the main gas valve to open.
If the sensor rod becomes coated with combustion byproducts, such as carbon or soot, the electrical signal is significantly weakened or blocked. The control board interprets this weak signal as a pilot light failure, even if the flame is visible. Consequently, the board keeps the main gas valve closed, preventing the main burners from igniting and avoiding the release of uncombusted gas. This is the most frequent cause of a lit pilot that refuses to initiate the heating cycle.
To address this, shut off the furnace power completely at the service switch before opening the burner compartment panel. The flame sensor is a thin, metallic rod positioned directly in the path of the pilot flame, usually held by a single screw. Carefully remove the sensor and gently clean the metallic rod using fine-grit sandpaper or steel wool. This removes the insulating layer of soot and oxidation, restoring its ability to conduct the necessary signal.
Airflow and High-Limit Switch Shutdown
Even with a clean flame sensor, the furnace may refuse to fire the main burner if an internal overheating condition is detected. This protection is managed by the high-limit switch, a safety device monitoring the temperature within the heat exchanger compartment. If the internal temperature exceeds a safe threshold, the limit switch interrupts the electrical circuit to the main gas valve. This prevents the furnace from sustaining a dangerous temperature that could damage the heat exchanger.
The most common cause of overheating is restricted airflow, which prevents heat from being properly transferred into the ductwork. A severely clogged air filter is the primary culprit, drastically reducing the volume of air the blower fan pulls across the heat exchanger. Homeowners should immediately check and replace the filter if it appears dirty or blocked, as this simple maintenance task restores proper air circulation.
Blocked return air grilles or closed supply vents can also contribute to airflow restriction and subsequent overheating. Ensure all vents are open and that furniture or drapes are not obstructing the flow of air. Once the underlying airflow issue is corrected, the high-limit switch automatically resets itself after the furnace cools down, allowing the normal ignition sequence to resume.
When to Stop Troubleshooting and Call a Pro
If the thermostat is calling for heat, power is on, the flame sensor is clean, and airflow is unrestricted, the problem requires specialized diagnosis. The issue has moved beyond simple maintenance into electrical or mechanical failure. One possibility is a failed main gas valve, an electromechanical device that should open when the control board receives the signal. If the pilot is lit but the main burner does not attempt to ignite, the valve may be faulty and require replacement.
Another complex failure point is the ignition control board, which manages the entire sequence of operations. A malfunction here can prevent the signal from reaching the gas valve, even if all safety checks pass. Similarly, a failing blower motor can trigger a shutdown that is difficult to diagnose without specialized tools.
Homeowners should never attempt to bypass safety controls, such as the limit switch or flame sensor. Any work involving the gas line, the main gas valve, or complex electrical components must be deferred to a licensed HVAC technician. Continuing to troubleshoot past these simple steps risks further damage or compromises safety.