There is neither a California statute nor any case decision which definitively answers your question. Nor do the California Rules of Court
governing a trial de novo after trial by written declaration address the issue.
In the absence of any specific statutory, decisional or court rule guidance on the subject, general principles of law will apply. Generally, there is no restriction on a judge hearing a retrial of a case which has been tried once (whether the first trial resulted in a hung jury or a conviction that was reversed on appeal). On the contrary, it is typically the case that the same judge will hear the case on the retrial (for reasons of judicial efficiency, if nothing else).
On the other hand, this also means that the procedures set forth in the California Code of Civil Procedure sections 170 through 170.9 for disqualifying trial judges will also apply. These statutes are too long to include here in their entirety, but you can read them at:
Section 170.1 provides grounds for disqualifying a judge for cause. It is not likely that any of these would apply to a request for trial de novo of a traffic citation.
However, section 170.6 allows any party to summarily disqualify one judge per case simply by declaring that the judge is prejudiced against the defendant. This statute also makes no specific mention of a traffic trial de novo, but does contain the following provision:A motion under this paragraph may be made following reversal on appeal of a trial court's decision, or following reversal on appeal of a trial court's final judgment, if the trial judge in the prior proceeding is assigned to conduct a new trial on the matter.
So, a 170.6 peremptory challenge can be used to disqualify the trial judge who has previously ruled in a case which has been reversed on appeal. Logically, the same principle would apply to a trial de novo.
Precise language acceptable for making a peremptory challenge is contained in subdivision (a)(5) of section 170.6.