How do you know if the thermostat is in an open position?

In cold weather, you'll notice that the engine temperature gauge registers only slightly or not at all, particularly if you're driving at highway speeds. You'll also notice that the temperature of your passenger compartment heater isn't nearly as warm as usual.

Your car will show a temperature if you allow it to idle for several minutes. The heat flowing into the passenger compartment will be warm as usual.

If your thermostat is open, the engine will rarely warm to a level you're used to seeing on the temperature gauge.