Cal. Code Civ. Proc. § 233
If, before the jury has returned its verdict to the court, a juror becomes sick or, upon other good cause shown to the court, is found to be unable to perform his or her duty, the court may order the juror to be discharged. If any alternate jurors have been selected as provided by law, one of them shall then be designated by the court to take the place of the juror so discharged. If after all alternate jurors have been made regular jurors or if there is no alternate juror, a juror becomes sick or otherwise unable to perform the juror's duty and has been discharged by the court as provided in this section, the jury shall be discharged and a new jury then or afterwards impaneled, and the cause may again be tried. Alternatively, with the consent of all parties, the trial may proceed with only the remaining jurors, or another juror may be sworn and the trial begin anew.
Ca. Civ. Proc. Code § 233