What should you do if a furnace won't ignite?

When a furnace won't ignite, checking the thermocouple and gas supply is crucial because these components are directly involved in the ignition process. The thermocouple serves as a safety device that ensures the gas valve is only open when there is a flame present. If the thermocouple is malfunctioning or faulty, it may not allow gas to flow, preventing ignition. Additionally, ensuring the gas supply is uninterrupted and that there are no blockages in the gas line is essential, as a lack of gas will prevent ignition altogether.

In contrast, checking the condition of the filter, while important for overall furnace operation, typically affects airflow and efficiency rather than immediate ignition issues. Increasing the thermostat setting does not resolve the root cause of the ignition problem; if the furnace is not functioning properly, simply raising the temperature will not assist in ignition. The option to remove and replace the furnace is an extreme and unnecessary step if the problem can often be resolved by diagnosing the thermocouple and gas supply issues.