i had the temp gauge problem jump up then return back then in the red knew it was a fault not an over heat as the rad hoses were not hot enough so in the process of changing and testing the coolant sensors.i bought the only one listed for my year model 230 2.8 1999 gets wrong sensor sent to me.it is a long probe type blue plastic around top 3 cm long
my sensors is a short fat ones but i cannot find them listed anywhere. my thermostat housing has 2 sensors 1 front and one rear the rear one has 10 cm wire sealed to it .
so here is the question what is this Rear sensor called as i cannot find any reference to it or part name i had a nightmare getting it out had to cut the wire to get a socket on it for removal.
is it poss my ducato hymer is a earlier model reg new 1999 but it is a after hymer Just a thought
regards

Hi, I have the same problem on an '02. 2.8 JTD, The guage is all over the place. from about 3/8ths to max temp. when running. The coolent pipes don't get over hot and there is no tell tail smell of hot coolant. I initially thought it was the fan switch (next to the coolant one) and changed that. I'm going to gange the guage switch on the engine and try that today. That said I've just done 130 miles with no over heat and the guage all over the place.

just been to the motor factors they are ate a loss nothing listed by there suppliers hinting to what this sender or switch is for or what it is. will have to replace the old one as is and hope for the best as i never marked the wires when i cut them thinking it was a temp sender
any help appreciated

If you have a dodgy temp gauge, mine went straight to normal running temp when the engine was cold, some go all over the shop.
Known issue apparently; I have a 2.8jtd 2004 but many are affected.
Solution for me, disconnect the negative battery lead then turn on the ignition a few times, maybe leave it on a few minutes to drain any static electricity.
Reconnect the battery and Bobs your uncle!

Hope it works for others