Under Los Angeles Rent Control section 151.11
A. A tenant may refuse to pay any rent in excess of the maximum rent or maximum adjusted rent permitted pursuant to this chapter ro regulations or orders adopted hereunder or as further permitted by this chapter or ordinance. The fact that such rent is in excess of maximum rent or maximum adjusted rent shall be a defense in any action brought to recover possession of a rental unit or to collect the illegal rent. This subsection shall not be applicable to any rental unit not subject to the restrictions on rent set forth in this chapter.
B. A tenant may withhold the payment of rent otherwise lawfully due and owing after July 1, 1979 until such time as the landlord has complied with Section 151.05 A. of this chapter. Once the landlord has complied with Section 151.05 A. of this chapter the tenant becomes obligated to pay the current rent and any back rent withheld pursuant to this subsection.
Here is section 151.05:
A. On or after July, 1979, no landlord shall demand or accept rent for a rental unit without first procuring, and serving on the tenant or displaying in a conspicuous place a valid written registration statement from the Department or its designee. On or after April 30, 1983, no landlord shall demand or accept rent for a rental unit without first serving a copy of a valid registration or annual registration renewal statement on the tenant of that rental unit.
