According to my 05 CTS Service Manual, "The transmission oil cooler system uses quick connect fittings throughout the system. Use a special tool to disconnect these quick connect fittings. Removing the transmission oil cooler lines without this tool will result in damage to the radiator, the transmission, and the transmission oil cooler caused by mixing the transmission oil and coolant or due to transmission oil loss.
Tools Required; J 38185 Hose Clamp Pliers
J 44827 Transmission Cooler Quick Disconnect