"The brake fluid tester pen measures the change of water content in brake oil based on the change in conductivity. It clearly shows the degree of water content through the LED. The high water content will cause the boiling point to decrease, affecting the braking performance, and also corroding the brake master cylinder and the sub-pump to cause rust and seizure. Red light indicates that the brake fluid needs to be replaced immediately, green light indicates good condition, and yellow light indicates that the condition is acceptable, but it is about to be replaced. Its operation is simple with accurate and fast testing. Specification: Product size: 150 x 25 x 25 mm/5.90 x 0.98 x 0.98"" Detailed description of the indicators: Green: The battery is OK, that is, the brake fluid does not contain water. Green/yellow: The water content in the brake fluid is less than 1%, the brake fluid performance is good, and you can use it with confidence. Green/yellow/yellow: The brake fluid has a water content of about 2% and the brake fluid can continue to be used. Green/yellow/yellow/red: The water content in the brake fluid is about 3%. It is recommended to replace the brake fluid. Green/yellow/yellow/red/red: The brake fluid contains at least 4% water, and the brake fluid needs to be replaced immediately. Troubleshooting: 1. LED flashes red, please replace the battery. 2. The lights all light on when the switch is turned on, please remove the battery and restart. 3. After the switch is turned on, release the button to measure. Tips: After each test, wipe the liquid part of the test point with a dry cloth or paper. Included: 1 x Brake Fluid Tester Pen"