As a strict matter of law, no, your prior DUI conviction is not a bar to certification. Of course, individual employers can still reserve the right to inquire about any criminal background you may have, and may also seek your permission to run a background check. It will be up to them to decide whether or not they feel that a prior DUI is a reason to deny employment, though I would suspect that a minimum offense DUI conviction from four years ago is probably not going to block all of your chances of employment.
It is unlikely that such a conviction would prevent certification. However, it is best to check with the professional occupational regulatory authority within your state. Also, please understand that an employer may refuse to hire you based on such a conviction as it could present a number of insurance issues for the employer.
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