Neb. Rev. Stat. § 18-2312

It shall be unlawful for any person to engage in business as an air conditioning air distribution contractor or to engage in the business of installing, altering, repairing, cleaning, adding to, or changing in any manner any air conditioning air distribution system or any furnace, any restaurant appliance hood or its duct system, or any other exhaust or intake ventilating system within a city or village having an air conditioning air distribution board or within the extraterritorial zoning jurisdiction of cities of the metropolitan class having such a board unless such person holds a certificate or is employed by a person, firm, or corporation holding such a certificate.