Brake fluid detection pen is based on a change in conductivity to determine the humidity change the brake fluid. This clearly shows the degree of moisture contained in the LED, a high moisture content will lead to a lower boiling point, affect the braking efficiency. Red indicates the brake fluid needs urgent replacement, green color symbolizes good condition, yellow shows the condition is acceptable, but soon replaced. check the car's quality oil brakes.5 LED display lights the percentage of moisture content in the brake fluid, as the test results are more accurate. Press the button to turn on, the green LED will light up.Insert the tester into the tank for a second, then the results will be indicated correctly using the LEDs. Testing POINT 3, DOT4, DOT5 brake fluid.Auto power off, Supports 1 LR03 AAA battery (not included) LED Indicator Display: 1. green LED = water content in brake fluid H2O Less than 1.5%, brake fluid qualified 2. yellow LED brake fluid indicators water content H2O = 1.5% to 3%, can continue to use, the test again after six months 3. LED red LED = water content in brake fluid H2O Greater than 3%, can not continue to use the brake fluid must be replaced Red LED: brake fluid replacement 4. red LED flashing: battery replacement 5. yellow LED: low moisture content, check again in six months 6. green LED: brake line fluid / mobile line Technical specifications: Green: Battery IN ORDER, that is, the brake fluid does not contain water Green / yellow: brake oil in water content less than 1%, better brake fluids performance, ease of use. Green / yellow / yellow: brake oil in water content about 2%, the brake fluid can continue to use. Green / yellow / yellow / red: brake oil in water content of about 3%, the proposed replacement of brake fluid. Green / yellow / yello