Pontiac 2006 GTO Owner's Manual page 215

The coolant level should be checked at each fuel fill, by
looking at the dipstick in the coolant overflow reservoir.
To remove the dipstick, push down on the coolant
reservoir cap while turning it counterclockwise.
When your engine is cold, the coolant level should be at
or above the bottom arrow on the dipstick. After the
vehicle has been driven and the engine is at normal
operating temperature, the level should be somewhere
between the two arrows on the dipstick.
If the coolant level is correct, replace the dipstick then
push down on the coolant reservoir cap while turning
it clockwise until it stops, to lock it into position.
Adding Coolant
If you need more coolant, add the proper DEX-COOL
coolant mixture at the coolant overflow reservoir, but
be careful not to spill it. Check the level with the dipstick
and keep adding fluid until the level is correct.
{
CAUTION:
You can be burned if you spill coolant on hot
engine parts. Coolant contains ethylene glycol,
and it will burn if the engine parts are hot
enough. Do not spill coolant on a hot engine.
When the level is correct, replace the dipstick then push
down on the coolant reservoir cap while turning it
clockwise until it stops, to lock it into position.
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