It is necessary to check the moisture content of your brake fluid because brake fluid is hygroscopic, meaning it readily absorbs moisture. Over a period of time moisture will cause brake fluid to degrade. Once brake fluid reaches a certain level the brake fluid is no longer able to function properly in the brake hydraulic systems. This Schwaben Brake fluid tester has a green, yellow and red light that represent's moisture content of DOT 4 brake fluid to determine if service is needed.Features:Green LED, H2O level is Yellow LED, H2O level is 1.5 to 3% =Replace soonRed Led, H2O level is >3% Replace ImmediatelyAuto-off functionBright Led Flashlight One AAA battery operation- not included