Why it’s bad: The LATCH system can make car seat installation easy, but it has its limitations. Many parents use both the LATCH system and the seat belt together, thinking more belts = more safety. This is actually false.
“The majority of seats on the market are designed to be installed with either lower anchors or the vehicle's seat belt. While using both may seem like a good idea, it may have a negative impact on the seat's performance in a crash,” says Kemp. Unless specifically specified by the manufacturer, the seat hasn’t been tested with both restraint systems in use. If both are in use during a crash, stress may be placed on the seat in ways not tested or designed for, potentially causing the seat to fail.
"Also, the LATCH system has a maximum weight limit that varies, and that weight is determined by adding the weight of your seat to the weight of your child. The max weight for a Honda Odyssey may not be the same as the max weight limit for a Nissan Sentra, and if you are over that weight and using the lower anchors, they could fail during a crash, releasing your child’s seat."
How to fix it: “Always follow the manual for your seat, and if you feel you aren't getting a safe install with one method, try the other.” Be sure to check both your car seat manual and your vehicle manual to see what the maximum weight limit for a specific LATCH system combination is, and follow it. When in doubt, schedule an appointment with a CPST to have your installation checked.