The Engine Coolant Temperature (or AIR) Sensor:     ( ECT is the modern SAE name)

It is just a resistor, it has no active electronics , like a MAF or MAP.  It's so very simple.
It's not precision.  (it's not an Instrument grade device)
The ECT usually shows 300 ohms hot, and reads (back probed)
0.6vdc.  HOT,180F hot.  See the math below.
The ECT/IAT both have the same resistance/heat curves, but are different in other ways.
One way to get bad reading from a good sensor  is to allow the ECT water level to drop below the probe tip.. (check cold radiator tank levels directly)
    The ECT  IAT Sensor.  
Old or alias names: (WTS = water temperature Sensor, or coolant temp. sensor)
The other sensor is the INTAKE Temperature Sensor or IAT. 
The IAT is very important on all 8v TBI motors.  (in fact, the 92 16v has this sensor missing, MPI)

The ECT and the IAT are really almost identical, in fact my 8 valve sensor both measure exactly the same.
The probe on the LEFT, above, is the sensor end, and inside is a simple resistor that that changes resistance with any temperature change.
It is just a device called a Varistor or thermistor (best name).  (variable + resistor)

To test it , you only need a simple ohm meter, a cheap digital ohm meter works great.  Select the 20k ohm range. ( or 2k range for hot)
The before starting engine temperatures vary by the local ambient temperatures and wind chill.
But on a warm morning, you can expect about 2400 Ohms .
Disconnected, this ECT/WTS sensor should read 2200  to 2700  ohms at 68 Deg. F (room temp) and reducing to near zero at a hot coolant of 180 deg F.
This is called a NTC, or a Negative Temperature Coefficient, Thermistor by technicians.  Rising temperature causes a falling reading.
2400 ohms is typical for normal room temp. water state.

It can be tested on the  engine with a voltmeter connected  or an ohm meter when its connector is disconnected.

This Device can be tested in a pan of hot or cold water, using the  simple ohm meter.
SEE HERE FOR IAT readings: (ohms and volts revealed.)
My  data, collected in my LAB. for both ECT and IAT sensors and voltage calc.'s.

The Fast test is on car.

In the morning before starting car , measure the resistance and it will match the ambient temperature.
The ECT will read about 2500 ohms , at 68F degress, or 6000 ohms when it is freezing outside 32deg. F.
Then start the car and get it good and hot, at least 180F.
The ECT will read, It will read 200 to 300 ohms.  ( if it reads higher , check the actual temperature of the coolant , and be sure it is 180F or more)
A high reading ECT will waste fuel.
If unsure then remove it and test it in a pan of ice water , then boiling water , then room temperature.

The IAT is best tested in a pan of water. (the air temp is constant, pretty much.)
The rules are the same.
Try Ice water and room temperature and then boiling water.

Removing the ECT will cause some loss of coolant. (but not the IAT)

The IAT is on the air cleaner box on a 16v engine 1993 and newer,, on the 1.8L it is on the Intake pipe.

The sensor is only $35 to $40.

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