Engine temperature sensors are typically located in or adjacent to the thermostat housing. To finf the housing follow the upper hose from the radiator, to where it connects to the engine. I do appreciate that this may be difficult in modern engines. Do not confuse intercooler, and coolant hoses.
Searching on Ebay for "Fiat Ducato Thermostat Housing" offers two different types of housing. One is metallic and seems to have a tapped hole for the sensor, with the coolant outlet connection in line with the thermostat. A second type offered appears to be a black plastic moulding with a right angle outlet, and a the sensor inserted in the side.
If the maze of pipes etc is similar to that on my 2.8jtd, you will have to remove some parts before you can access the housing and sensor.
eLearn for the x250 is available in the downloads section, and there may be some info in the procedures section, however to use eLearn you will need a Windows computer.