Hi @wyattpung
Looking at the [https://www.centurytool.net/880_460_666_Wire_Speed_Control_Assembly_p/880-460-666.htm|control board] for the welder, perhaps check if the potentiometer is OK.
Disconnect the power and then remove the board to gain access to the pot and use an Ohmmeter to see if the resistance of the pot varies as you turn it. Hopefully there may be some markings on it to help identify its value etc
[image|3033551]
(click on image)