Car seats are made of plastics. Consider what happens to a plastic toy if it is left outside for some time. The plastic becomes brittle and can develop cracks when stressed. Car seats are subjected to extreme heat and extreme cold while sitting in your vehicle, so the plastics eventually react just like that toy left in the sun.


You may not be able to see that the plastic is breaking down, or is more brittle, but that change could be dangerous in a crash when the car seat shell is stressed. You can see this problem in action by watching this crash test video of an expired car seat. In the video, the car seat harness breaks through the shell of the seat upon impact. This car seat would not have adequately protected a child in a crash. It's far safer for parents to buy a new car seat than to take a chance on a car seat that may be too old to function properly in a crash.