There are several checks you can do but you would need a scan tool.
You could go to Auto Zone (AZ) or O'Reilly's (OR) and for FREE they can pull the codes to the car. Most important: Once they check your codes, if they find something and you don't get it fixed and need to get back with us, please make sure you tell us exactly what the code was, number and all. Example, if the code was E0568 O2 Sensor bad. Then make sure you give us all of that. While there for FREE also they can bring their tester out and check your battery and alternator.
If there were no code then disconnect the coolant level connector and see if the light goes out. If it does and you have coolant, then probably the sensor is bad and needs replacing. It is located in the bottom of the surge tank. I have attached info for you.
SURGE TANK REPLACEMENT
J 38185 Hose Clamp Pliers. See Special Tools.
1. Raise and support the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
2. Drain the cooling system. Refer to Draining and Filling Cooling System (L61).
3. Lower the vehicle.
Fig. 6: Identifying Low Coolant Sensor Electrical Connector
Courtesy of GENERAL MOTORS CORP.
4. Disconnect the low coolant sensor electrical connector.
5. Remove the low coolant sensor electrical harness from the retainer.
Monday, February 16th, 2009 AT 12:31 AM