Temperature Gauge Sending Unit The sending unit is located in the front section of the intake manifold in the vicinity of the thermostat.
The temperature sensor (sends coolant temp to the PCM) is located in the top of the thermostat housing. The temperature sender (sends coolant temp to the gauge) is located in the bottom front of the same housing.
A low coolant level could cause the temp gauge not to read correctly. A faulty temp gauge sending unit could also be the cause. The coolant leak, you would have to find the source of the leak to determine the cause.
Low on coolant, sticky thermostat, loose connections, faulty sending unit.
There is a temp sensor on the water inlet for the PCM, and sending units for the gauge on the side of the heads. Normally part stores don't carry the sending units.
If the wiring and connections at the coolant temp sending unit check okay start with replacing the coolant temp sending unit. Temp sending unit should be less than $20.
The 95 Lumina with a 3.1 engine does not have a temperature sending unit. Instead, the dash temp gauge is controlled by the engine coolant temp sensor. This sensor is located on the driver's side, top of the engine.
The temp sending unit may be shorted to ground. Unplug the sending unit to see if the gauge drops to cold. If it does, you have a faulty temp sending unit.
A faulty temp sending unit can do that.
Could be, low on coolant (a leak in the system), a stuck closed thermostat, a faulty temp gauge sending unit, a bad head gasket or cracked head (worst case scenario).
First try unplugging the coolant temp. sending unit and plugging it back in(may just be bad connection at the plug), next replace the temp. sending unit if the plug-in thing didn't work.
To fix the temp gauge check the temp sending unit first. A bad sending unit is a common problem with the temp system.