If a technician determines that a natural gas furnace has a defective heat exchanger, what should the technician do?

When a technician identifies a defective heat exchanger in a natural gas furnace, it is critical to prioritize safety. A defective heat exchanger can lead to dangerous conditions, such as carbon monoxide leaks, which pose serious health risks to occupants. By informing the homeowner of the problem and turning off the gas supply, the technician takes immediate action to prevent further operation of the furnace, mitigating the risk of carbon monoxide buildup or other hazards related to a malfunctioning appliance. This step ensures that the homeowner can seek repairs or replacement without exposure to potential dangers, emphasizing the technician's responsibility to safeguard both the property and its residents.

The other actions, such as continuing to operate the furnace or attempting repairs without proper safety measures, could exacerbate the situation, making them unsafe choices. Replacing the entire furnace could be unnecessary and more costly if the issue is specifically with the heat exchanger and can be resolved through replacement or repair.