The coolant needs corrosion inhibitors to protect the different metals in the engine from attack by corrosion. These inhibitors are gradually consumed. To ensure that the coolant maintains a sufficiently high inhibitor content, new inhibitors must be added. They are provided by the Volvo coolant filter which contains inhibitors which are dissolved when the coolant passes through the filter. The filter and coolant must contain exactly the same inhibitors. Volvo's coolant always has the correct inhibitor content.