[pjmoreland]

Here's an internal schematic of a stock temperature gauge.

There's one winding (62-Ohm) that pulls the needle toward C. Its power runs through the external resistor.

There's another winding (48-Ohm) that pulls the needle toward H. Its power runs through the temperature sensor.

Reasons for the needle staying too far toward C:

1) Temp sensor resistance too low
2) Damaged 48-Ohm winding
3) External resistor resistance too low
4) Top terminal of gauge shorted to ground

Reasons for the needle moving too far toward H:

1) Temp sensor resistance too high
2) Damaged 62-Ohm winding
3) Damaged external resistor
4) Poor connection between bottom end of external resistor and ground
5) Passenger side terminal of gauge or sensor wire shorted to ground

Personally, I have seen the following issues:
-Gauge terminals shorted to the gauge cluster housing due to poor alignment between the terminals and the gauge cluster tin holes
-Damaged external resistor (infinite resistance)
-Damaged gauge internal windings
-Sensors that have their resistance go way too low as a result of the current flowing through them (this is difficult to test for because the resistance is correct when the wire is disconnected)
-Reproduction gauges that just don't behave like an OE gauge