Here's a good way to test the coolant temperature sensor.
First Unplug the sensors electrical connector.
And then using an ohmmeter, measure the resistance value of the coolant temperature sensor by hooking up one of the meter's leads to one of the terminals on the sensor electrical connector, and the other lead to the other terminal on the sensor electrical connector.
On vehicles with old, single-wire sensors, hook up the meter leads to the terminal on the connector and the sensor's body (ground) to take your reading.
Check your vehicle service manual for the correct resistance value for your ECT sensor. However, not all service manuals have this information.
Most sensors of this type have a resistance value of 3000 ohms or more at about 55F (13C).
Make a note of the sensor's resistance.
Now you're going to take another pair of readings.
This time, start the engine and let it idle.
Set the transmission to Neutral and apply the parking brakes.
Wait for about one to two minutes and measure the engine temperature and the sensor's resistance as you did before.
Make a note of these new pair of values.
Without turning off the engine, wait for about one to two minutes and repeat this process again.
Take another pair of readings in about one to two minutes again, always noting the value readings.
Then turn off the engine.