Do not leave the engine running when you check the radiator fluid level, and do not ever try to check the radiator fluid level when the engine is hot.
Place your pointer finger and middle finger from one hand together and press down on the cap while simultaneously turning the cap off with your other hand. This will prevent a coolant burst in the event that the system is still pressurized.
Do not add fluid while the engine is still warm.
You should test the coolant protection level in the spring and fall, and more often if you drive the car under extreme conditions.
