talkingcars

Looking at your profile picture I am guessing that your car is a mk1 VVC but it would be useful if this detail is included as the advice can differ.

As a mk1 it will only be OBD1 so the ECU can only be read be read by a few scanners such as the www.pscan.uk
As a VVC it will have a MEMS 2.0J ECU.

The K series engine has 2 coolant temp sensors. One has a single lead and is for the dash temperature. To test it earth the wire and the gauge will go to the top.

The more important sensor is the one with two wires. It tells the ECU the coolant temperature and thus control fuelling and ignition. If it is telling the ECU that the engine is warm the ECU will not supply enough fuel and the ignition will not have the correct advance. The only way to see what temperature the sensor is seeing is to plug in an OBD reader and read the coolant temp.

If the ECU gets no signal from the coolant sensor it assumes the engine is too hot and it will run the radiator fan. To check this unplug the sensor and the fans should run.