Upon arrival, I conducted a thorough inspection of the 10-year-old Lennox furnace. During the assessment, I identified a critical flaw in the heat exchanger, which poses a safety risk if not addressed. I promptly informed the customer about the issue and explained the potential dangers associated with operating the furnace in its current state. I then presented several options for resolving the problem, including the replacement of the heat exchanger to ensure safe and efficient operation of the furnace. The customer appreciated the detailed explanation and chose the recommended solution.