California Code, Health and Safety Code - HSC § 41755

(b)(1) Source testing of engines for compliance purposes shall not be required more frequently than once every three years, except where evidence of engine tampering, lack of proper engine maintenance, or other problems or operating conditions that could affect emissions from the engine are identified.

(2) A district may conduct source testing to determine compliance with mass emission limits where there is an indication of noncompliance.

(3) Except as required for purposes of paragraph (2), source testing of engine emissions for compliance purposes shall not be required of engines for which there is no applicable emission limit.