Does a dui arrest pending arraignment, pretrial, and trial, appear on an employers background check?
It might, depending on the thoroughness of the background check.
Employers usually can't ask applicants about arrests that did not result in a conviction; however, there's an exception that allows them to ask if you have a case pending. California Labor Code §432.7
Answer any pending questions truthfully. If your employer finds out you lied in response to a direct question about your pending case, you could be terminated at any time.
The arrest, yes. If and when convicted, that will show.