Looking at the article, thats a stupid way to replace the fan. buying a 5 volt 40mm fan instead of routing a new fan into the printer power mains would have been trivially easy to put inline. instead they have you mess with the main power block and pull all the pins out of the pin block at once and hope you get them back in the right spot?
You most likely have one or more wires going into the incorrect spot at this point. Probably the thermistor leads and the fan leads swapped, and may have fried your thermistor . it could be something else if you are lucky. You are probably going to have to trace out the wires and make sure that the wires you think you are using are actually the pair you are supposed to be using. The Ohai instructions may help some https://ohai.lulzbot.com/group/a16b1f2d-5c1f-47fe-a83d-9d68cca5d62c/