For 1990-2009 cars

1. Verify that the cooling system is properly filled. Refer to Cooling System Draining and Filling .
2. If the complaint is a low temperature reading, check the temperature gage for proper operation. Connect a scan tool to the vehicle and read the system temperature. Compare this reading to the temperature. Replace the gauge or switch if necessary.
3. Refer to Heating Performance Diagnostic if the complaint is no heat from the HVAC system.
4. Verify proper thermostat operation. Refer to Thermostat Diagnosis .