1 x Genuine Wanhao replacement thermocouple. These run from the Duplicator 4X/S motherboard up the loom and down onto the hotend where they are secured using an allen key. The thermocouple is one of the most sensitive parts of any 3D printer which can sometimes fail. This is a cure for the "Heater Error！My temperature reads are failing!..." message you sometimes get on your Duplicator 4 screen