If you have an older furnace, you may still have a model that relies on a pilot light for ignition. Manufacturers have entirely phased out pilot light ignition from modern furnaces, but many homes still use older models with these systems. Efficiency is the primary disadvantage of pilot lights; these systems are otherwise fairly reliable and easy to understand.

Of course, the relative inefficiency of pilot lights is no reason to stop using an otherwise operational furnace, so you're probably looking for solutions if your light won't stay out. The good news is that a pilot light that won't light (or won't stay lit) usually comes down to a few issues. This article will discuss three common reasons your furnace's pilot light can fail.

1. Thermocouple Issues

Thermocouples are simple temperature sensors that serve a critical safety function in any furnace with a pilot light. Since your pilot light burns continuously, it also needs a continuous supply of gas for fuel. Under normal circumstances, this fuel burns away, and the combustion products escape through the flue. However, a failed flame can potentially create a small but dangerous gas leak.

The thermocouple prevents this by cutting the gas flow whenever a flame isn't present. When you hold the button to light your furnace, you're waiting for the thermocouple to generate enough voltage to hold the solenoid open. If the thermocouple is faulty or the rod is dirty, the pilot light will usually go out almost immediately when you take your finger off the ignition button.

2. Solenoid Issues

The other side of the pilot light equation is the solenoid in the gas valve. Solenoids are relatively simple electromechanical devices that convert electrical energy into mechanical force. In the case of the solenoid in your furnace's gas valve, an electrical signal from the thermocouple controls a plunger that allows or disallows the gas flow.

An internal problem with the solenoid can stop the plunger from pulling down when it receives sufficient voltage from the thermocouple, keeping the gas valve closed. In these cases, the button (which connects directly to the solenoid plunger) that you use to light your pilot light will usually pop back up, and the pilot light will go out immediately.

3. Pilot Tube Issues

Finally, the pilot tube is another potential source of trouble. The pilot tube delivers gas to the pilot light burner, providing the fuel necessary to keep the flame going. Since pilot lights are relatively small, they don't need much fuel to stay lit. As a result, pilot light tubes are typically fairly small, making them vulnerable to clogs.

An issue with your pilot tube can prevent your pilot light from igniting. Unlike faulty thermocouples or solenoids, a clogged pilot tube will stop the pilot light from lighting since there's no fuel available. Since this tube is essentially a (very small) piece of gas plumbing, you'll want a professional to diagnose and repair any issues with your pilot tube.

Reach out to a furnace repair contractor to learn more.