All modern gas furnaces follow a relatively similar startup sequence. You can think of this sequence as the checklist that pilots must follow before taking off. The furnace's control board systematically checks each safety sensor, confirming that the unit is ready to start. It will also need to prove certain events, such as correct exhaust pressure and burner ignition.

These steps are necessary to protect the furnace and prevent the potential for dangerous situations, such as exhaust gas leaks. A failure at any step can leave the furnace in an unsafe state, so the control board must be able to recognize these situations and stop the ignition. However, these many steps can make figuring out why your furnace won't light a little complex.

What's Involved in Diagnosing an Ignition Failure?

You might think that ignition failures are relatively easy to recognize. If your furnace doesn't light, there must be a problem with the ignition system or gas supply. Unfortunately, the ignition system isn't even always the first culprit. When your furnace's burners don't ignite, the only thing you can know for sure is that there was a failure somewhere in the startup sequence.

Modern furnaces usually include small LEDs or displays to provide fault codes when something goes wrong. However, these fault codes rarely offer a foolproof diagnosis. The fault code can point a technician in the right direction, but it's almost always necessary to perform additional troubleshooting. Blindly replacing parts based on these fault codes is often expensive and rarely works.

Instead, most technicians will start by investigating the sensor reported by the fault code. Since these codes can only describe failures the control board can see, there will always be one or more sensors to blame. Finding the actual underlying cause of the problem usually requires carefully checking that sensor and its electrical connections and looking for other sources of trouble.

Why Shouldn't You Attempt to Diagnose Ignition Problems Yourself?

In addition to the potential complexity involved in troubleshooting an ignition fault, it's easy to waste a lot of money blindly throwing parts at a problem. If you aren't sure where the underlying fault is, you may replace working sensors or even more expensive parts of your furnace. In a worst-case scenario, you can cause more harm and need additional repairs.

Another consideration is that ignition problems sometimes result from issues with the gas supply. A clogged or damaged gas line can prevent the burners from igniting or cause an inconsistent flame. These problems require a trained expert to diagnose and repair them since working with a residential gas line can be very hazardous.

If your furnace isn't lighting, it's always worth trying a few simple steps, such as changing your air filter and ensuring adequate airflow through your return ductwork. However, if these steps don't solve the problem, it's time to call in a professional.

For more information, reach out to a local service, such as Myers Heating & Cooling.