1. Take some coolant from the expansion tank using a hydrometer (see arrow in illustration). The hydrometer float will sink to a certain depth depending on the density.
2. Read the coolant density on the scale to determine at what temperatures freeze protection will be provided. Some hydrometer models have a pointer indicating the degree of freeze protection. In most cases, this test will reveal that the concentration of antifreeze in the coolant is insufficient to provide adequate freeze protection. For this reason, antifreeze should be added to the coolant. A rough rule of thumb is that approximately ¼ liter of antifreeze increases freeze protection by approximately 10°C.
3. Remove the engine splash guard.
4. Place a suitable container under the vehicle to collect the coolant.
5. Remove the drain plug on the bottom of the radiator or remove the lower coolant circulation hose from the radiator and drain 1-2 liters of coolant.
6. Replace the hose and secure it, or screw in the drain plug.
7. Add the appropriate amount of undiluted antifreeze and pour in the drained coolant.
Information obtained from this resource AUDImanual