At nearly every car seat inspection I conduct, I find at least one aftermarket accessory that could reduce how well the seat protects the child in a crash. Parents buy these products with good intentions — wanting their child to be more comfortable, warmer, or entertained — but the fundamental problem is the same across all of them: no aftermarket accessory has been crash-tested with your specific car seat.
Car seat manufacturers spend years engineering and crash-testing their seats with specific materials, harness geometry, and padding. Adding anything that didn’t go through that testing process introduces an unknown variable into a system designed to protect your child at 30+ mph impact forces. Here are the eight most common offenders I see, and why each one is a problem.
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1. Aftermarket Harness Strap Covers
These padded fabric covers wrap around the harness straps near your child’s shoulders and neck. Parents buy them because the harness can feel rough against a baby’s skin, especially in summer. The problem: added thickness between the harness and your child’s body creates slack. In a crash, the harness compresses through the padding before it contacts your child — meaning your child moves farther forward before the restraint engages.
The other issue is material friction. The manufacturer designed the harness webbing to grip against your child’s clothing at a specific coefficient of friction. A slippery or fluffy cover changes that equation and can allow the child to slide beneath the harness in a crash. For a deeper dive on this specific topic, read our complete guide to car seat strap cover safety.
What to do instead: If the harness irritates your child’s skin, check that the harness height is correctly adjusted. Dress your child in a thin cotton onesie or shirt with a collar that protects the neck from webbing contact. Only use strap covers that came with your car seat from the manufacturer.
2. Seat Belt Positioning Clips and Adjusters
These devices reposition the vehicle seat belt across a child’s body, typically pulling the shoulder portion lower or the lap portion tighter. They’re marketed for older kids who have moved to booster seats or are transitioning to seat belts alone.
The danger: any device that changes the seat belt’s routing path alters how crash forces are distributed across your child’s body. A clip that moves the shoulder belt off the neck might redirect it across the chest in a way that concentrates force on the sternum or ribs. A lap belt pulled too tight in the wrong position can cause internal abdominal injuries.
What to do instead: If the seat belt doesn’t fit your child properly, they still need a booster seat. Use the five-point seat belt fit test: feet flat on the floor, knees bent at seat edge, lap belt across upper thighs, shoulder belt across middle of chest, and child can sit fully back against the seat. If any point fails, keep using the booster. Check our safest booster seat guide for recommendations.
3. Vehicle Seat Protector Mats
These mats go under the car seat to protect your vehicle’s upholstery from indentations and scratches. I see them at almost every inspection, and they cause more installation problems than any other accessory. The mat creates a slippery layer between the car seat base and the vehicle seat, allowing the car seat to shift during normal driving — and especially during a crash.
Several car seat manufacturers explicitly state in their manuals that nothing should be placed between the car seat and the vehicle seat surface. Using a mat may void your car seat’s warranty and, more importantly, compromise the tightness of your installation.
What to do instead: Accept that some upholstery indentation is normal and temporary. If you’re concerned about long-term damage, place a thin cotton towel (not a thick mat) under the seat — some manufacturers permit this. Check your specific car seat’s manual for guidance.
4. Aftermarket Infant Head and Body Support Inserts
These cushioned inserts are designed to cradle a newborn’s head and body inside a car seat that feels too large. The problem is positioning: an aftermarket insert can change the angle of your baby’s head and airway, push the baby higher or lower in the seat than intended, and alter how the harness contacts the body.
For newborns, airway positioning is critical. A baby whose chin touches their chest can experience positional asphyxiation — restricted breathing caused by head position rather than external obstruction. An insert that tilts the baby’s head forward even slightly increases this risk.
What to do instead: Use only the infant insert that came with your car seat. If your seat didn’t include one and your newborn seems too small, roll up thin receiving blankets and place them along the sides of your baby (not behind the back or under the body). This provides lateral support without altering the harness path or recline angle.
5. Bundle Me and Winter Car Seat Covers
These zip-up covers keep babies warm by wrapping around the entire car seat, with the harness straps poking through slits in the fabric. The issue: any material between the harness and the child creates the same slack problem as strap covers, but across the entire body. The harness appears snug over the puffy cover but is actually inches away from your child’s torso. In a crash, the cover compresses and the child submarines beneath the harness.
Some “shower cap style” covers that go over the car seat canopy without interfering with the harness are acceptable, but even these need careful evaluation to ensure they don’t affect airflow or harness accessibility.
What to do instead: Remove your child’s coat before buckling the harness. Tighten the harness snugly over thin clothing, then place the coat backward over the child like a blanket, or use a car-seat-specific poncho that has a thin back panel and a warm front panel. The harness should always be snug against the child’s body with no more than one finger of space at the collarbone.
6. Attachable Mirror Mounts
Back-seat mirrors let you see your rear-facing baby from the driver’s seat. They’re incredibly popular and many parents consider them essential. The safety concern: in a crash, an unsecured mirror becomes a projectile traveling at the speed of impact. A mirror that weighs even a few ounces can cause injury when it’s launched across the back seat.
Additionally, mirrors that clip onto the vehicle headrest can interfere with the headrest’s crash protection function. And the distraction factor is real — taking your eyes off the road to check a mirror defeats the purpose of keeping everyone safe.
What to do instead: If you use a mirror, choose one with a shatterproof surface and a secure attachment that’s been tested for breakaway at crash forces. Accept that you won’t be able to see your baby during every moment of the drive — that’s okay.
7. Car Seat Handle Toys and Toy Bars
Toys that clip or strap onto the car seat handle or harness are designed to keep babies entertained during drives. In a crash, these become projectiles — and a hard plastic toy traveling at 30 mph can cause serious injury. Even soft toys can obstruct a child’s airway if they’re thrown into the child’s face during impact.
Toys attached to the harness straps are especially problematic because they add weight and bulk to the harness system, potentially affecting how the harness performs during a crash.
What to do instead: For babies, short car rides don’t need entertainment. For toddlers on longer rides, offer soft fabric toys or books that can rest in their lap. Nothing should be attached to the car seat itself.
8. Aftermarket Sunshades That Attach to the Car Seat
These canopy extensions or clip-on shades attach to the car seat to provide additional sun protection beyond what the built-in canopy offers. They can interfere with the car seat’s canopy mechanism, affect airflow around your child, and become an obstruction or projectile in a crash.
What to do instead: Use your vehicle’s window shades (the kind that suction-cup to the window glass) or apply window tint to rear windows. These protect from sun without attaching anything to the car seat.
The Rule to Remember
If it didn’t come in the box with your car seat, or isn’t sold by your car seat’s manufacturer specifically for your model, don’t put it on or in the seat. That’s the simplest way to ensure every component of your child’s restraint system has been crash-tested together.
For car seats with manufacturer-approved accessories, check our Britax accessories guide. And for help making sure your seat is installed correctly without any risky add-ons, visit a NHTSA-certified car seat technician for a free check.
