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Visible soot is a common indicator of incomplete combustion, particularly with propane. When combustion is incomplete, not all the fuel is fully oxidized, leading to the formation of solid carbon particles. This results in soot, which may be seen as black deposits around the combustion area or in the exhaust.

In contrast, clear water vapor is typically produced during complete combustion when propane reacts properly with oxygen, resulting in carbon dioxide and water vapor without any particulate matter. High exhaust temperature can indicate efficient combustion as more fuel is converting to heat energy, while low gas pressure may not directly relate to combustion efficiency but could instead indicate a supply issue. Overall, the presence of visible soot is a clear and direct sign that combustion is not proceeding as intended, thus pointing to inefficiencies in the burning process of propane.