The absolute most critical part of using a car seat is the in-built harness. It's utterly paramount to ensure that it is appropriately adjusted for your child. A poorly, loosely fitted in-built harness can lead to much greater forward movement of the child’s torso and head in frontal accidents, can allow them to roll out of the car seat in side impact collisions, and in extreme cases can result in ejection from the car seat entirely.
The harness should be nice and snug, with little give – the 'Pinch Test' is the best way to gauge if your child's harness is tight enough. We have a great video about doing the Pinch Test here.
The harness should be adjusted each and every time you put your child in the restraint, as slightly different seated positions, different clothing and nappies can all significantly alter the ‘size’ of the child in the harness, and extra special care should be taken in the colder months (see our post on how winter coats can affect harness operation) to ensure you're getting a nice, snug fit.