A combination car seat is a forward-facing-only car seat that has a 5-point internal harness and can also be used later as a belt-positioning booster car seat.
Combination car seats are often mistakenly called convertible car seats or booster seats. However, unlike a convertible car seat, combination seats cannot be used rear-facing. Unlike booster-only seats, the combination car seat has a harness that is used for children under a certain weight, but can also be used in booster mode later on.
Since combination car seats can only be used forward-facing, these seats are designed to be used by older toddlers and big kids. Kids under two years old should still be riding in a rear-facing convertible car seat or a high-weight infant car seat. If your older toddler still fits in a rear-facing car seat, that is the safest choice.
How to Find the Best Combination Car Seat
Lots of inexpensive combination car seats have a harness weight of about 40 pounds. However, there are also many combination car seats that have harnesses rated beyond 40 pounds. It's easier than ever to keep your child in a harnessed car seat to 80 pounds or more. Allowing your child to remain in a 5-point harness longer is a big safety benefit. The harness spreads crash forces over more of the child's body, offering better protection versus a lap and shoulder seat belt.
When shopping for a combination car seat, look for a reasonably tall shell and the highest harness strap slots you can find, to go along with that nice, high weight limit.
The tall shell and high harness slots will allow your baby to use the car seat longer, since your child's ears will need to stay below the top of the shell and the harness straps must be above the child's shoulders when riding forward-facing.
A car seat with a foam liner is also a good choice for your child.
These impact-protection liners are made of bicycle helmet-type foam, and they offer the best side-impact crash protection. You'll need to look under the car seat cover to see how much impact foam is on the seat. Having the foam around your child's head is most important, and a liner that also extends down the sides of the car seat is a nice bonus.
Other Harness Features
Since you'll need to move the harness straps up as your child grows, take a look at the adjustment mechanism before buying. Make sure you're comfortable changing the harness height. Many of today's models have no-rethread harness straps that move up and down along with the head wings, or that can slide up and down on tracks within the car seat. These models can usually be adjusted without un-installing the car seat. Other models require you to unbuckle or un-LATCH the seat to adjust the harness height. Be sure you can replace the harness properly every time, so that it can adequately protect your child.
Thick, wide harness straps are another nice feature to look for in a combination car seat.
Lots of economy-priced seats have thin straps that twist easily. Twisted straps cannot do a good job of keeping your child safe in a crash. If the car seat you like doesn't have thick, twist-resistant harness straps, you will need to take extra care to smooth the straps each time you use the car seat.
Booster Mode Features
Wide shoulder belt guides will be helpful in booster mode, because the slit-type shoulder belt guides found on many older combination car seat models don't allow the shoulder belt to slide freely, possibly allowing dangerous slack in the seat belt. This is one area where it's helpful to try out the combination car seat in your vehicle before you buy. Buckle the car seat in as though you'll be using it as a booster. Check to see if it's easy to buckle that way, because by the time your child is riding in a booster, chances are they'll want to do the buckling on their own. Does the shoulder belt slide easily through its guide?
When it comes to booster mode, LATCH may also be a handy feature. Some combination car seats may allow the use of the lower anchors in booster mode. You'll need to check the instruction manual to see if this is OK. If so, go ahead and LATCH it. That will keep the seat from flying around in a crash when your child isn't riding with you. If your combination seat can't be used with LATCH in booster mode, try to make a habit of re-buckling the booster when your child gets out of the car.
Pay Close Attention to Harness Weights
The most common combination car seats can be used in the harnessed mode until your baby reaches 40 to 50 pounds. Then you can switch to booster mode by removing the harness straps and using the seat with your vehicle seat belt as a belt-positioning booster through 80 to 100 pounds (or even more, depending on the car seat). If a car seat box says that the seat can be used from 20-100 lbs, or 20-80 lbs, check the labels on the sides of the car seat, or the instruction manual, to find the maximum harnessed weight. With combination car seats, parents often assume that the harness can be used to the maximum listed weight for the seat, not realizing that the upper weight limits are for booster mode only. Using an internal harness beyond its intended maximum weight could result in car seat breakage in a crash. That could leave your child at serious risk of injury.