I arrived for a new customer fan running all the time problem. I inspected the 46 year old furnace and found the cause of the system failure is the fan high limit control is sticking and will not shut off after the flame shuts off from the satisfied call from the thermostat. I can manually shut the fan off but the fan will continue to run after each heat cycle. I also inspected the heat exchanger and found the 2nd cell from the right has excessive rust buildup and is in far worse condition than the other 4 cells. I preformed a carbon monoxide test and found no readings at this time. I made this aware to the homeowner and recommended to replace the system due to the condition and age of the furnace. He only wanted to replace the fan/high limit control and the fan relay that controls the fan in cool and on/auto mode. I do not have the fan/high limit control and will have to track down the correct parts and return to repair the furnace. The homeowner agreed to replace both parts and agree