Pontiac 2005 GTO - Manual (Page 218)

To add coolant to the overflow reservoir, do the
following:
1. Remove the coolant overflow reservoir cap when
the cooling system is no longer hot.
2. Fill the coolant overflow reservoir with the proper
mixture, until it reaches the bottom arrow on the
dipstick.
3. Replace the dipstick, then press down on the
coolant reservoir cap while turning it clockwise until
it stops, to lock it into position.
If there is still no coolant visible on the dipstick, there is
one more thing you can try. You can add the proper
coolant mixture directly to the radiator, but be sure the
system is cool before you do it.
{ CAUTION:
Steam and scalding liquids from a hot cooling
system can blow out and burn you badly. They
are under pressure, and if you turn the radiator
pressure cap — even a little — they can come
out at high speed. Never turn the cap when the
cooling system, including the radiator pressure
cap, is hot. Wait for the cooling system and
radiator pressure cap to cool if you ever have
to turn the pressure cap.
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