Which of the following is a common troubleshooting step if a furnace does not ignite?

When troubleshooting a furnace that is not igniting, inspecting the oil line for air is a crucial step, especially in oil-fired heating systems. Air in the oil line can prevent proper fuel delivery to the burner, which is essential for ignition. Ensuring a continuous flow of fuel is vital for the furnace to operate correctly. If there's air trapped in the line, it might disrupt the fuel supply and lead to ignition failure.

Other steps, while potentially helpful in diagnosing various issues, do not directly address the ignition problem as effectively. Checking duct insulation pertains to ensuring efficient heat distribution rather than directly affecting ignition. Cleaning the thermostat could help with temperature regulation but doesn’t directly influence whether the furnace ignites. Examining the furnace exterior may provide some insights into overall system health but is less likely to uncover the specific issue related to ignition. Thus, inspecting the oil line is a targeted approach that addresses a key element of the ignition process in oil-burning furnaces.