Fitting a child car seat – common problems

Child car seats

Fitting a child car seat – common problems

By Lisa Galliers

Article 3 of 4

Making sure your car seat is fitted properly is vital to keep your child safe. Use our handy reference guide to spot if anything could be wrong.

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Car seat fitting issues

If you have trouble fitting your car seat, you're not alone.

In our 2015 survey of more than 1,400 UK parents, we asked what were the most common issues they'd had when fitting or using their car seat. By far the biggest issue was a twisted seatbelt. 

All of these issues can easily be identified and corrected with regular checks of your car seat. Read on to find out how to spot common issues and the danger your child could face.

Car seat issues
Issue %
Twisted seat belt 26%
Seat isn't stable in the car 7%
The harness isn't tight enough 6%
Hard to attach Isofix connectors 5%
Child escapes from car seat 5%
Support leg doesn't reach the floor 2%
Car seat doesn't fit in my car 1%

Table notes
1 Based on 1,476 parents surveyed in March 2015.

Signs your child car seat is fitted wrongly

Even if you choose a Best Buy child car seat that fits your car properly and have it installed by an expert, you still need to do regular checks to make sure it's fitted properly. 

A correctly fitted car seat will help to provide the best crash protection possible for your little one and reduce the risk of serious injury. 

Babies and children are more vulnerable to injuries in a car crash because their skeletons aren't fully developed and some of their muscles are weak. 

Best child car seats for babies – top tips of what to look for when choosing your baby's car seat.  

Our gallery below highlights some of the common problems that show a child car seat is wrong for your child or your car.

1. Adult seatbelt under the child's arm

When a child is seated in a booster car seat, the adult seatbelt should come across the child's shoulder and lap. In the pictures below the seatbelt is under the armpit. In a car crash the seatbelt would fail to reduce movement in the upper body, which could lead to injury.

2. Incorrect position of adult seatbelt at neck

In the pictures below, the position of the adult seatbelt indicates the child car seat is unsuitable. The seatbelt should come across the child's shoulder and chest, but the seatbelt cuts across the neck.

The belt should be positioned across the collarbone, which is stronger, to reduce the risk of injuries.

Find reviews for more than 100 child car seats, covering baby car seats, Group 1, Group 2/3, i-Size and Isofix car seats in the Which? Child car seat reviews

Not a Which? member? Sign up today for a £1 Which? trial and access all our expert reviews of the latest child car seats. 

3. Adult seatbelt not on shoulder

A suitable child car seat would place the adult seatbelt across the child's shoulder, rather than over the upper arm (as in the first and third image), or not touching the shoulder at all (as in the second image). These seatbelt positions reveal these child car seats are unsuitable because the child may not be held in properly by the seatbelt in a crash.

4. Child car seat harness too loose

A child car seat harness should fit snuggly across your child's body, with space for two fingers between the harness and the child.

Some child car seats have harnesses to act as restraints. For a harness to work well, it needs to fit snugly across your child's body – you should be able to fit just two fingers between the harness and your child's body.

In our images, there's too much space between the child and the harness, leaving room for the shoulders to slip out and the child would be thrown too far forwards in a collision.

5. Incorrect position of adult seatbelt across the tummy

With a suitable child car seat, the adult seatbelt should come across the child's lap. This is because a child's pelvis is not as strong as an adult's and can't absorb the force of a crash.

If the seatbelt sits across the child's tummy, as in the images below, the seatbelt could crush the soft internal organs, including the stomach and liver, and cause serious internal injuries.

6. Adult seatbelt too slack

A child could be thrown out of the seat in a crash if the seatbelt or harness isn’t correctly adjusted.

It's important that the adult seatbelt is tightly fitted so that it securely keeps the child and car seat in place. In the first image below, the seatbelt being slack across the lap could mean too much force is applied to the chest by the seat belt, crushing the lungs. Or the child car seat could move in a collision.

In the second image, the belt is so slack across the child's shoulder that it wouldn't stop her moving forward during a crash.

Always make sure the seatbelt is properly clicked into its buckle and that harnesses are secured tightly.

7. Adult seatbelt twisted

If a seatbelt is twisted, it won't react as it should in a collision and it may in fact fail to work altogether. As you can see from our images below, sometimes seatbelt twisting can be obvious or subtle. 

When fitting your car seat, make sure that the seatbelts run smoothly and securely through any fittings. The same applies if the car seat uses the seatbelt to hold your child in place.

8. Poor positioning of adult seatbelt

Sometimes the problems with fitting a child car seat can be quite difficult to notice. In these two examples, there are different problems with the fitting of the seats that commonly occur.

In the first picture below, you can see the car seat is unsuitable for the child because of the position of the adult seatbelt. Although the belt may have been correctly routed, for this child it wouldn't give any protection as it does not secure her over the strongest parts of her body (her shoulder, across her chest and across her lap). She's simply too small for this kind of seat.

In the second picture, the problem with the seatbelt mainly occurs because the seatbelt comes over the arm rest of the child car seat (just before it's buckled). This part of the seatbelt should be underneath the arm rest to better secure the child in the best position to protect her.

Buckle crunch

Positioning the car’s seatbelt buckle hard up against the child seat frame can cause the buckle to fail under crash conditions.

Only the seat belt webbing should be in contact with the frame of the child car seat. If the buckle of the adult belt lies across the frame of the child seat, pressure on the buckle (in an accident, or even under sharp braking) could cause the buckle to fail. If it fails, the buckle is likely to open, allowing your baby or child to be projected out of the seat, completely unrestrained.

Check your car seat today

Keep your little ones as safe as possible by carrying out a few simple checks on a regular basis. 

Download our quick reference guide to checking your car seat - 10 essential car seat checks 

And check these other essentials:

  • Seat belt Check it's long enough to secure the child car seat, and make sure you can feed the seat belt through the slits in the child car seat without obstructions.
    It should be easy to adjust and secure the car seat's harness and should leave enough space for your child to stretch their legs.
  • Movement If fitted properly, your car seat should have minimal forward or sideways movement. When you open the buckle of the adult belt, the car seat should spring upward slightly.
  • Buckle crunch Only the seat belt webbing should touch the child car seat's frame, not the buckle. Otherwise pressure on the buckle could make it fail, meaning your child won’t be securely restrained in an emergency situation. Check the child car seat’s fitting booklet for more information.
  • Airbags Disable the airbag before fitting a rear-facing seat. If the airbag goes off, the force can be fatal for a baby.
  • Car seat accessories Don’t place anything beneath the child car seat to protect your car seats (a blanket, for example).

For more expert safety advice, see our guide to car seat accessories.