GMC Cadillac CTS 2004 Owner's Manual page 324

If you have to add coolant more than four times a year,
have your dealer check your cooling system.
Notice: If you use the proper coolant, you do not
have to add extra inhibitors or additives which claim
to improve the system. These can be harmful.
Checking Coolant
The coolant surge tank and pressure cap are located on
the driver's side of the vehicle, toward the rear of the
engine compartment. See Engine Compartment
Overview on page 5-12 for more information on location.
5-30
CAUTION:
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Turning the surge tank pressure cap when the
engine and radiator are hot can allow steam
and scalding liquids to blow out and burn you
badly. Never turn the surge tank pressure
cap — even a little — when the engine and
radiator are hot.
The vehicle must be on a level surface. When your
engine is cold, the coolant level should be at the
FULL COLD/FROID line on the side of the surge tank.
Follow the arrow from the top of the tank down the
side to the horizontal mark.
If the Check Coolant Level message in the DIC
comes on and stays on, it means you're low on
engine coolant. See DIC Warnings and Messages
on page 3-70 for more information.