The brake fluid detection pen is based on the change of conductivity to determine the moisture change of the brake fluid. It clearly shows the extent of moisture contained by the light-emitting diode, high moisture content will lead to a lower boiling point, affecting the braking performance. Red indicates that the brake fluid needs to be replaced immediately, green represents a good condition, yellow indicates that the condition is acceptable but soon replaced. 1. Green LED lights = water content in brake fluid H2O < 1.5%, qualified brake fluid 2. Yellow LED lights = water content in brake fluid H2O = 1.5%-3%, can continue to use, test again six months later 3. Red LED lights = water content in brake fluid H2O > 3%, can not continue to use the brake fluid needs to be replaced Red LED: change the brake fluid 4. Red LED flashing: replace the battery 5. Yellow LED: low moisture, recheck in six months 6. Green LED: brake fluid lines / cell row Features: Five LED indicators display the test results more accurately. Test DOT 3, DOT4, DOT5 brake fluid Press the switch to turn on the device, the green LED light that represents the correct battery lights up, the green light does not light up on behalf of replacing the battery Automatic shutdown Features: Status: new Material: Plastic, Electronic Components Color: Black Length: Approx. 155mm / 6.1in Width: 18mm / 0.7in Power Supply: 1.5V LR03 AAA Battery (Not Included) Note: The data are all measured manually, please allow 1-3cm error Due to differences in light and display, there may be some color differences, please refer to the actual item received.