Tex. Transportation Code § 524.039

(a) Not later than the fifth day before the date of a scheduled hearing, the person who requested a hearing may apply to the State Office of Administrative Hearings to issue a subpoena for the attendance of the breath test operator who took the specimen of the person's breath to determine alcohol concentration or the certified breath test technical supervisor responsible for maintaining and directing the operation of the breath test instrument used to analyze the specimen of the person's breath, or both. The State Office of Administrative Hearings shall issue the subpoena only on a showing of good cause.

(b) The department may reschedule a hearing once not less than 48 hours before the hearing if a person subpoenaed under Subsection (a) is unavailable. The department may also reschedule the hearing on showing good cause that a person subpoenaed under Subsection (a) is not available at the time of the hearing.