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If a freeze plug is leaking then it will need to be replaced. Depending on where the plug is at components may need to be removed to access the plug. The coolant will need to be drained, and once the plug can be accessed then it will be driven out of the hole. The new plug can then be driven in either with a freeze plug tool or with an appropriate sized socket. If the plug that needs to be replaced is in an area that's tough to get to then a freeze plug installation kit would be the way to go, it comes with different sized adapters for different plugs and different handles to allow you to drive the plugs in from different angles. If you do use a socket make sure to use one slightly smaller than the inside diameter of the plug, otherwise the socket will be stuck in the plug as it shrinks when driven in.