What does a significantly high operating vacuum reading indicate?

A significantly high operating vacuum reading is an indication of obstacles in the fuel flow system. This typically signifies that fuel is having difficulty moving through the supply line, which could be due to a blockage in the fuel filter or a kink in the oil supply line.

When the operating vacuum reading is high, it suggests that there is greater resistance in the system, which impedes the normal flow of oil. Essentially, this high vacuum reading is a sign that either the fuel filter is clogged with debris, necessitating a replacement or cleaning, or that the oil supply line has been compromised, preventing the necessary oil flow to the burner. Therefore, option C directly addresses the importance of monitoring vacuum levels to maintain the efficiency and functionality of the heating system.

The other options do not correctly reflect the implications of a high operating vacuum. Proper oil flow and a clean oil supply line would typically indicate low or normal vacuum readings, while excessive oil pressure relates more to the supply and combustion aspects than to vacuum readings.