On Friday, May 10, 2019 at 11:19:34 AM UTC-7, Paul Agnew wrote:
91.215
(c)Transponder-on operation. While in the airspace as specified in paragraph (b) of this section or in all controlled airspace, each person operating an aircraft equipped with an operable ATC transponder maintained in accordance with § 91.413 of this part shall operate the transponder, including Mode C equipment if installed, and shall reply on the appropriate code or as assigned by ATC.

You need to check the paragraph (b) mentioned above. Not all controlled airspaces (in the US) require the use of a transponder, even if it is installed.