Theory of Operation
WARNING: This page is about a different car, the 2011 RAM Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.
The thermostat rationality diagnostic runs during a drive cycle that starts with the coolant temperature below 140°F. This diagnostic looks at the how cold the system is at start up, what the inlet air temperature is, and the amount of fuel burned during that drive cycle. The PCM estimates what the coolant temperature should be. If the coolant temperature is not above a calibrated threshold, the rationality fails. The PCM assumes that the thermostat is stuck open.