GMC T-Series 1999 Owner's Manual page 219

Coolant Recovery Tank (Gasoline Engines)
The vehicle must be on a level surface. When your
engine is cold, the coolant level should be at or above
COLD FULL level on the coolant recovery tank.
See "Lights, Low Coolant Warning Light" in the Index
for further information.
yellowblue
If this light comes on and
stays on, it means you're
low on engine coolant.
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