Thermostat is stuck open, not allowing engine to reach operating temp. Replace the thermostat. could also be your heater core needs cleaning also check your rad cap to make sure that it holds the pressure
If your question is more "why" and not what to do: Your engine gets hot and needs to be cooled. To do this there are coolant passages throughout your engine and they connect to your radiator which uses outside air to dissipate the head from the coolant before returning back to the engine. Your water pump circulates the coolant through the system. Your thermostat is like a door that holds the coolant in the engine back until it's hot enough to go to the radiator to be cooled. If it is stuck open the coolant will be continually circulated through the radiator and continually cooled... it will not allow your engine to "warm up"... all it is doing is preventing your coolant from getting up to operating temperature when it's stuck open. the way your heat works is it has a heater core, a mini radiator, under your dashboard. it has coolant running through it and when you turn your heat on it blows air through this heater core. if your thermostat is stuck open then your coolant won't be allowed to get hot so it won't be hot when it goes through the heater core with the fan blowing through it.
A malfunctioning thermostat can cause your temperature gauge to go up and then allow the gauge to go all the way down. When the thermostat is stuck closed, your engine will overheat and water will not get to your heater core. When the thermostat opens the engine cools down and water flows to the heater core.
If your engine thermostat sticks open it will run cooler on your temperature gauge and you won't see the temperature gauge go up to the point where your engine thermostat opens and then the temperature gauge will show a drop in temperature , your check engine light will probably come on , your gas mileage will be less , the heat from your heater probably won't get as hot If the engine thermostat were to stick in the closed position your engine will overheat ( but there are several other causes for an engine to overheat )
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