Which of the following is a sign of improper gas combustion?

A yellow flame is a clear indication of improper gas combustion. In an ideal combustion process, natural gas should burn with a blue flame, which indicates that there is sufficient oxygen for complete combustion. The presence of a yellow flame suggests that the combustion process is inefficient, often due to a lack of oxygen. This incomplete combustion can lead to the production of carbon monoxide, a dangerous and potentially lethal gas, as well as soot and other byproducts. Identifying a yellow flame is crucial for ensuring safety and efficiency in gas appliances, as it signals the need for immediate attention to the burner and ventilation to restore proper conditions for combustion.