Feature:Staying safe and knowing your car is in better condition is the most important. Ideally, brake fluid should be checked regularly and changed every two years. When moisture seeps into the brake system through the brake pipes and seals, the brake fluid will become contaminated and the risk of brake failure will increase. This pen-shaped brake fluid tester can easily help determine if it is time to change your brake fluid.LED lights indicate the percentage of water in the brake fluid:Green: OnGreen/Yellow: Less than 1%Green/Yellow/Yellow: Approx. 2%Green/Yellow/Yellow/Red: Approx. 3% (Time to change the brake fluid.)Green/Yellow/Yellow/Red/Red: Approx. 4% (Change brake fluid immediately!)Please note:- This brake fluid tester is calibrated for DOT3 DOT4 brake fluid, used in most vehicles manufactured since 2006.- Replace the battery if the green LED light no longer illuminates after pressing the button.- Wash the test probe with tap water after each use.- In case of skin contact with brake fluid, wash skin or clothing thoroughly with water.- Do not expose brake fluid to the atmosphere for a long time as it is very hygroscopic.