There are a few ways to tell if your car's thermostat is failing. One of the most common symptoms is that your car's engine temperature will fluctuate frequently or will not reach normal operating temperature. You may also notice reduced engine power, poor fuel economy, or overheating. Additionally, you may notice that your car's heater is not working as well as it should. If you're experiencing any of these symptoms, it's important to have your car's thermostat checked by a mechanic to ensure that it's functioning correctly.