Can a sheriff reposses a car in California?

Of course, it is called a "writ of replevin" and happens daily. I will cut and paste information on this subject:

In creditors' rights law, replevin, sometimes known as "claim and delivery," is a legal remedy for a person to recover goods unlawfully withheld from his or her possession, by means of a special form of legal process in which a court may require a defendant to return specific goods to the plaintiff at the outset of the action (i.e. before judgment). In other situations, a party seeking relief may elect to adjudicate the right to possession prior to obtaining immediate relief to obtain the property in question. In such cases, replevin actions are still designed to afford the petitioning party a relatively speedy process for obtaining judgment, as compared to typical lawsuits. The summary remedy afforded by replevin statutes can be thwarted by defendants who contest the claimant's right to possession, by contesting the plaintiff's complaint, and insisting on traditional litigation involving discovery, and in some cases, trial by jury.

Replevin actions are often filed by secured creditors seeking to take possession of collateral securing loans or other debt instruments, such as retail installment contracts. A common example is where an automobile finance company initiates a replevin action to gain possession of a vehicle, following payment default. Replevin actions are usually employed when the lender cannot find the collateral, or cannot peacefully obtain it through self-help repossession. Replevin actions may also be pursued by true owners of property, e.g., consignors seeking return of consigned property that the party in possession will not relinquish for one reason or another.