2001 Dakota Thermostat Keeps Failing

I have a 2001 Dodge Dakota Sport 4.7L V8 and the thermostat keeps failing. My father has put at least 2 thermostats on the truck in the past 2 years but they continue to fail as I have just gotten a code on my OBD2 reader that the thermostat stays open for too long leading the engine to not reach optimal operating temp. What could be the problem?