You probably already know how important transmission fluid is for your automatic transmission, but one thing many people aren’t sure of is when to change transmission fluid. Transmission fluid acts as hydraulic fluid, lubricant, and coolant for your car, but that doesn’t mean it lasts forever. Transmission fluid has a shelf life too just like any other fluid in your car, and breaks down over time from heat, use, and contaminants. If left alone, it can start to cause erractic behavior with your transmission such as shifting issues, slipping, and overheating.