Well I reread your question and do think that your coolant temperature sensor has to be the problem. I would ask though, how is the heat in your van? Is it blazing hot? Lukewarm? Or non-existent? Hot heat indicates a working thermostat, and tells us that the problem likely electrical (sensor, wiring, or gauge). Non-existent heat indicates a thermostat that is stuck open (or possibly an unrelated HVAC problem, but we will assume that that system works fine for our purposes). Lukewarm heat likely indicates that the thermostat is opening too early, but is opening and shutting in a decent manner.
A coolant temperature sensor is a thermistor. Basically, as it warms up, it conducts electricity with lower resistance. The PCM supplies a base voltage, and then judges coolant temperature based on the amount of resistance in the sensor (as measured by the lower signal voltage being returned to the PCM by the sensor). So, using a multi-meter and removing the sensor, you can heat it up and then measure the resistance in it to see if it is accurate. I am attaching a list of resistances as they translate to coolant temperatures.
Images (Click to enlarge)
Tuesday, January 23rd, 2018 AT 7:15 PM