1. Unplug the furnace.
2. Remove the cover on the very top of the furnace.
3. Disconnect the thermocouple you would like to replace (the high limit thermocouple is connected to cable 403B, while the control thermocouple is connected to cable 408B). See image below.
4. Replace the thermocouple (Part number: 101043). Spares can be obtained from Rapidia.
5. Wire the thermocouple. Remember, black is positive and red is negative for Type S thermocouples.
6. Reinstall the cover.
7. Plug the furnace back in.