if youve ever run a vehicle without a thermostat you would know that the thing will never warm up and it gets hot spots. the thermostat closes when the coolant temp gets to a certain temp and opens when the temp is high enough. so one that is stuck open or sometimes not closing would keep the engine from warming up. however, a bad CTS could still be the problem but a thermostat is cheap and fairly easy to replace. how is the heat in the car?