Most systems have carbon as the final stage. This carbon stage absorbs residual iodine. Once the carbon has absorbed all it can hold it will allow the taste/odor of iodine to pass through to your water supply. Once you notice this taste shift it is time to replace the purification cartridge. It is still purifying the water at this point but you will want to install a new cartridge as soon as possible in order not to exceed the effective life of the cartridge. The sediment stages of the systems should be replaced whenever their rated capacity is reached or when they have collected enough sediment to reduce the water flow.