What symptoms indicate that a cad cell may be dirty?

A dirty cad cell, which is a critical component in an oil burner system responsible for detecting the presence and quality of the flame, can significantly impact burner performance. When the cad cell is dirty, it may not adequately sense the flame, leading to the burner not starting at all. This is because the oil burner control relies on feedback from the cad cell to confirm that combustion is occurring safely and effectively. If the cad cell cannot detect a flame due to soot or contaminants obscuring its surface, the safety mechanism within the burner will trigger to prevent operation, resulting in the burner failing to start.

Flame flickering can indicate other issues with the burner, such as unstable combustion or airflow problems, but it does not directly confirm the state of the cad cell. Increased combustion noise and unusual odors during operation could suggest different mechanical problems rather than a specific failure related to the cad cell. Hence, the inability to start the burner is the most direct symptom of a dirty cad cell, making it the correct answer in this context.