What is the test for determining whether a company can be held liable for negligence in a motor vehicle accident?

Youtz v. Thompson Tire Co., 46 Cal.App.2d 672, 116 P.2d 636, makes reference to the comment (just cited) from the Restatement. It quotes from a leading case on the particular point (at pages 676-677, 116 P.2d at page 639): "But in the other class of cases, where the article itself is not inherently or intrinsically dangerous to health or life, a third party, seeking to hold the maker liable for injuries suffered by him in the use of the article, must show that the maker knew it was unsafe and dangerous, and either concealed the defects, or represented that it was sound and safe. But even when this is shown, the maker will not be liable, if it is made to appear that the purchaser had knowledge of the defects at and before the third party was injured in using it. * * *" Among other reasons for the rule exculpating the maker from liability, the cited case holds, is the fact that 'the maker's wrongful act in such a case is not the proximate cause of the injury, when it is shown that there was the intervention of a new agent, to wit, the purchaser, who, with knowledge of the danger used and permitted others to use the article."