2005 Ford Escape 4-cyl is giving a P0128 code, which can be caused by a bad sensor or a bad thermostat, according to my internet searches.
The temp gauge on the dashboard reads the same as it always has when the car has been running on the road for about 15 minutes, not quite midway up the gauge.
I asked the dealership to diagnose the problem. They say they ran a "pinpoint test" and identified it as a stuck thermostat.
My question is, what steps must be taken to rule out the temperature sensor as the problem and to identify the thermostat as the thing that's malfunctioning here?