What is the first step in troubleshooting a gas furnace that is not igniting?

The first step in troubleshooting a gas furnace that is not igniting is to check the gas supply to the furnace. Ensuring that gas is available and flowing to the furnace is crucial, as a lack of gas indicates a fundamental issue that can completely prevent ignition. Before investigating other potential issues such as the filter, thermostat settings, or pilot light operation, confirming that the gas supply is intact allows for a clear and logical approach to diagnosing the problem.

If there is no gas supply, the furnace will not ignite regardless of other conditions. Therefore, this step is essential. Once the gas supply is confirmed as operational, you can then proceed to examine other elements that might affect the furnace's performance.