Often, no — and this is one of the most important features of product liability law. California is a strict-liability state for defective products, which means you generally do not have to prove that a manufacturer was careless. You have to prove that the product was defective and that the defect caused your injury.
That is a meaningful advantage over an ordinary negligence claim, where you must show a failure to use reasonable care. In a strict-liability case, even a company that was careful can be responsible if its product was defective — because the law places the cost of injuries from defective products on those who make and sell them.
Product claims can also be brought on negligence and breach-of-warranty theories, and they are often pleaded together. But the strict-liability path, focused on the defect rather than fault, is frequently the strongest, which is why identifying and proving the defect is the heart of the case.