Q: My car is not warming up to operating temperature

Air pockets in the cooling system can cause all kinds of weird behavior. So first of all, be sure the cooling system is completely full by running the car in park with the heater on max and the radiator cap off for a few minutes, or until you feel heat coming out of the vents. The excess air should escape from the radiator opening. Now if the thermostat is the correct one, there is no good reason why the car shouldn’t be warming up. There are a few subtle differences in thermostats that might mean the wrong one would fit in the spot, but not function correctly, but since you are buying original equipment BEHR I presume that you have the right part. That makes me think you may have a problem with instrumentation. Maybe your car is actually warming up and the gauge is not reading correctly. This is easy enough to check with a non-contact thermometer, which is a really cool tool you can buy for about 20 dollars. Just point it at the thermostat outlet after the car has been running for about 15 minutes and it will tell the story. The only other possibility I can think of is that some of the heater and bypass hoses are routed incorrectly so that they allow the coolant to circulate when the thermostat is closed. But that’s difficult to say without seeing the car. If you contact Your Mechanic, they can send a technician to your home or office to [check out your cooling system]https://www.yourmechanic.com/services/heater-is-not-working-inspection) and tell you what it’s going to take.