Lexus GS450h Owner's Manual page 541

NOTICE
When adding engine/power control unit coolant
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Add coolant slowly after the hybrid system has cooled down sufficiently. Adding cool
coolant to a hot hybrid system too quickly can cause damage to the hybrid system.
To prevent damage to the cooling system
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Observe the following precautions:
Avoid contaminating the coolant with foreign matter (such as sand or dust etc.).
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Do not use any coolant additive.
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GS450h_300h_OM_OM30F92E_(EE)
7-2. Steps to take in an emergency
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