The temperature sending unit is part of the sensor on the car that determines the temperature of a certain location and then sends this information to the computer modules located in the car, which respond by showing results on the thermometer gauge and changing engine functions to match the new temperatures. Typically the sensors are located near or on the engine to give a good idea of engine heat, but the sending units are located closer to the gauges they operate.
Basic Sending Unit Diagnosis
Sending units can be difficult to diagnose, since they sit somewhere between the sensors themselves and the temperature gauges (they may even be part of the sensors themselves, making diagnosis even more difficult). How do you know if the problem lies in the gauge itself, the sensors or the sending unit? You can always check the sensors themselves---one of the most common problems with temperature sensors located near the engine is a disconnection caused by vibration, which dislodges the sensor. If the sensor is dislodged, you can fix this problem and the sending unit should operate well.
You can also tell if the sending unit may be faulty by observing the gauge itself. Is the gauge constantly stuck on average or high temperatures? Then the problem is probably the thermostat sensor. Is it refusing to show a temperature at all? Then the problem probably lies in either the gauge or the sending unit itself.
Testing the Sending Unit
To be completely certain the sending unit is faulty, you will need to test it separately. First, locate the sending unit you want to test. There are typically three in a normal car engine: the one that shows engine temperature will be located on top of the engine block (usually), and the one that shows water temperature will be located on the front of the engine block. Sending units tend to have only one wire leading out, while sensors tend to have three, which can help you determine which is which.
You can then detach the sending unit from the gauge and connect its wires to an ohmmeter to observe its reaction. Since the sending unit is made with a thermistor that sends an electrical signal based on temperature changes, the current it produces will show if it is operating correctly. Carefully place the unit in a container of hot water and observe the readouts of the ohmmeter. If the current begins to flow, lower to higher, then the unit is fine. Otherwise, either the unit or the wires you are testing are faulty.
If you replace the sending unit, make sure that you find a replacement that matches your vehicle model specifications.