The vehicle must be on a level surface when checking the coolant level.
It is normal to see coolant moving in the upper coolant hose return line when the engine is running.
Check to see if coolant is visible in the coolant surge tank. If the coolant inside the coolant surge tank is boiling, do not do anything else until it cools down.

If coolant is visible but the coolant level is not at or above the mark pointed to, add a 50/50 mixture of clean drinkable water and DEX-COOL coolant.
Be sure the cooling system is cool before this is done.
What to Use
How to Add Coolant to the Coolant Surge TankLeaving the Vehicle with the Engine Running (Automatic Transmission)
Warning
It can be dangerous to leave the vehicle with the engine running.
It could overheat and catch fire.
It is dangerous to get out of the vehicle if the shift lever is
not fully in P (Park) with the parking brake firmly set. The vehicle can roll.
Do not leave the vehicle when the engine is ...
OnStar Additional Information
Transferring Service
Press to request account transfer
eligibility information. The Advisor can assist in canceling or removing account
information.
Selling/Transferring the Vehicle
Call 1-888-4-ONSTAR immediately to terminate your OnStar services if the vehicle
is disposed of, sold, trans ...
Radio Frequency Statement
This vehicle has systems that operate on a radio frequency that complies with
Part 15/Part 18 of the Federal Communications Commission (FCC) rules and with Industry
Canada Standards RSS-GEN/210/220/310, ICES-001.
Operation is subject to the following two conditions:
The device may not cause ...