The cold junction modification adds a temperature sensor to measure the temperature at the board. The new firmware uses this to correct the temperature readings from the thermocouples and make the heating process more accurate.
Cold junction compensation page:
Unexpected Maker video:
Note that Seon has his oven on the side so he doesn’t have to detach the cables. However, if you put the top to the left side of oven (as opposed to the right as he has it) the cables reach (at least they did for my oven) and you can leave the oven flat.
The temperature sensor I used can be found here:
When you read the datasheet it tells you that you need a 4.7kOhm resistor as well. I used these:
If you use a different temperature sensor, make sure it meets the requirements of the new firmware and be sure to check the data sheet for wiring.