Yes, transmission fluid can go bad if it sits too long. Over time, transmission fluid can deteriorate due to exposure to heat and oxygen, which can cause it to lose its ability to lubricate and protect the transmission's moving parts. As a result, the transmission may experience excess wear and tear, decreased performance, and even failure in severe cases. It's recommended to change the transmission fluid as per the manufacturer's recommendations, usually at intervals of 30,000 to 60,000 miles, depending on the vehicle and driving conditions. If the vehicle has been sitting unused for a long time, it's a good idea to have the transmission fluid checked and changed if necessary to ensure proper performance and longevity of the transmission.