GMC 2009 Sierra - Manual (Page 341)

Cooling System
When you decide it is safe to lift the hood, this is what
you will see:
A. Coolant Surge Tank
B. Coolant Surge Tank Pressure Cap
C. Engine Cooling Fan
{ CAUTION:
An electric engine cooling fan under the hood
can start up even when the engine is not running
and can injure you. Keep hands, clothing, and
tools away from any underhood electric fan.
If the coolant inside the coolant surge tank is boiling,
do not do anything else until it cools down. The vehicle
should be parked on a level surface. Check the
coolant level after the system cools down. Some
amount of coolant can be lost due to overheating.
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