First, check your engine coolant level. If it is low, you need to find out why. If the coolant level in the radiator is full, it could be many things including a bad thermostat, a clogged heater core, or a broken or damaged air temperature door or actuator.
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Checked the coolant level and it is not low. How do I check the other things?
To check the other things, it really requires some specialized equipment. The cooling system operation needs to be verified for proper operation first. This involves accessing the data stream in your onboard Engine Control Module (ECM) and comparing the Temperature Sensor data to readings taken with a pyrometer or infrared thermometer to ensure that the sensor is providing the correct information to the ECM. Once this is verified, the thermostat operation can be verified by using the thermometer or pyrometer and the ECM data to determine at what temperature it is opening. If this all checks out ok, then you move to checking the inlet and outlet temperatures of the heater core to see if it is possibly clogged up. If this is ok, then you move to the inside of the vehicle to verify mode door and door actuator function. Your Saturn may be equipped to by-pass some of this because the ECM or the Climate Control Module might have fault codes present which will lead you to the cause. For instance, there may be an "Engine Not Reaching Operating Temperature" fault code, in which a failed thermostat is almost always the cause, or the Climate Control Module may have a fault code that indicates a mode door actuator problem.
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Already checked coolant level. I was not low. How do I check the other things?
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