Combination booster seats, which have a forward-facing internal harness plus the ability to be used as a belt-positioning booster with the vehicle seat belt, are very popular choices among parents who hope to get more use for their car seat dollars. Many of these combination booster car seats are labeled on the package with the total weight range, such as 30 to 100 pounds. However, many parents never read the instruction manual or side car seat labels to learn that the total weight range does not apply to the internal harness.
Some internal car seat harnesses are made to be used to 65 or 80 lbs., but a large percentage of combination booster seats are designed to be used without the internal harness after the child weighs 40 pounds. Carefully read the instructions or side labels to be sure the harness in a combination booster seat is removed at the maximum harness weight. The booster seat should be used with a lap-shoulder vehicle belt at that point.
When an internal harness is used past the recommended maximum weight, the child is at risk for serious injury in a crash. Maximum weight recommendations are made via crash testing. If a booster seat doesn't perform well with the harness above a certain weight in crash tests, it will not perform well in real-life crashes with higher speeds and other variables. The biggest risk in using an internal harness past the maximum harness weight is that the harness straps could pull through the booster seat frame in a crash, allowing the child to be ejected from the seat.
If your child is in a harnessed car seat that also performs as a belt-positioning booster seat, please take a moment to read the side labels on the booster seat. These labels should define the maximum weights for both the internal harness and the belt-positioning booster seat.
Heather Corley is a certified Child Passenger Safety Technician.