GMC Duramax Diesel 2015 Owner's Manual page 103

10-24 Vehicle Care
If there is still no sign of steam and
the vehicle is equipped with an
engine driven cooling fan, push
down the accelerator until the
engine speed is about twice as fast
as normal idle speed for at least
five minutes while the vehicle is
parked. If the warning is still there,
turn off the engine and get everyone
out of the vehicle until it cools down.
The decision may be made not to lift
the hood, but to get service help
right away.