Toyota PRIUS Manual page 496

4-3. Do-it-yourself maintenance
 Coolant selection
Only use "Toyota Super Long Life Coolant" or a similar high quality ethylene
glycol based non-silicate, non-amine, non-nitrite, and non-borate coolant
with long-life hybrid organic acid technology.
"Toyota Super Long Life Coolant" is a mixture of 50% coolant and 50%
deionized water. (Minimum temperature: -35 C [-31 F])
For more details about coolant, contact any authorized Toyota dealer or
repairer, or another duly qualified and equipped professional.
 If the coolant level drops within a short time of replenishing
Visually check the radiator, hoses, engine/power control unit coolant reser-
voir caps, drain cock and water pump.
If you cannot find a leak, have any authorized Toyota dealer or repairer, or
another duly qualified and equipped professional test the cap and check for
leaks in the cooling system.
CAUTION
 When the hybrid system is hot
Do not remove the engine/power control unit coolant reservoir caps.
The cooling system may be under pressure and may spray hot coolant if the
cap is removed, causing serious injuries, such as burns.
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