People are unaware that the PCM controls and functions radiator fan(s) differently on a C6 and how it is done with a C5.

The C5 has 2 fans, C6 has one.

C5's PCM controls one or 2 relays to get both fans to run at a low or higher speed rotation.

As seen below the PCM fan tables only allows the fans to be running below 30-35 MPH and shown stock the low and high speeds rotation on between 201 and 219 water degrees

If the Tstat is replaced with like a 160 degree then in tuning we could command the fans to come on sooner like from 160 to 180 F deg.

The C6 is different in it has ONE fan.

If you notice the fan table in the C6 PCM ( upper part of image below) the lowest temp where fan can even be controlled is 87 C or 189 def F.

Then as the temp increases above that the PCM controls the amount of rotation of the fan.

As seen in stock calibration the fan is not even turning until 91C or 195F and even then the fan is only turning 15% rotation and not until 105C (225F) is fan rotating 90% of it's ability.

With the C5 then it is clear you could set the fan to come on at a much colder temp but in the C6 you cannot force fans on below the coldest temp of that table of the 87C no matter how much cooler T-stat is installed.