GM Service Manual Online
For 1990-2009 cars only
  1. Verify that the cooling system is properly filled. Refer to Cooling System Draining and Filling .
  2. If the complaint is a low temperature gauge 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 gauge. Replace the gauge or switch if necessary.
  3. Refer to Heating Insufficient in HVAC Systems with AC - Manual if the complaint is no heat from the HVAC system.
  4. Verify proper thermostat operation. Refer to Thermostat Diagnosis .