http://www.benplace.com/engine/trouble_parts.jpg
the blue one (forget the name temp1 on the black one, old picture, ididn't
know better at the time), the black one is for the gauge in the dash, the
blue one is located on the thermostat housing in front of left hed on 2.1L:
http://www.benplace.com/engine/temp_sender_2.jpg

http://www.benplace.com/85rouge/rouge111.jpg
the blue one again on Digijet 1.9

All wasser FI need one, they are coolant temp sender, they tell the ECU at
what temp the coolant is, you also have a temp sender at the AFM. a third
gauge for the FI system is the OS sender, it's on the cat on 2.1L and on the
J pipe on 1.9L. An OS can be unplug in case of bad running, ECU goes in sfe
mode.

Now, to your question, just pull on it and you will see. As oppose to the OS
sender, the FI can NOT run without a coolant temp sender (or in some case,
will run POORLY. The sign on a bad one are rough running to not running at
all, heavy smoke (of all color, he he he) at the exhaut. Some time, it's
close to  bad fuel regulator.

I have seen semi bad one (that is why i have seen strange smoke color)!!!
that part is under 15$, can be change pretty quickly on 1.9L an with a
coolant shower on 2.1L.

Ben