Immerse probe of the tester into brake fluid to gain the following readings GREEN LED =Brake fluid H2O < 1.5% - Brake fluid OK / Battery OK YELLOW LED =Brake fluid H2O < 1.5 - 3 % -Low percentage of water, Change within 6 months RED LED = Brake fluid H2O > 3 % - Change Brake Fluid. Tester has Auto-Off Function