Gov. Code, § 21432

If, prior to attaining the minimum age for voluntary retirement for service applicable to members of his or her class, a recipient of a disability retirement allowance, other than one for industrial disability, engages in a gainful occupation not in state service, the board shall reduce his or her monthly disability retirement pension to an amount that, when added to the compensation earned monthly by him or her, shall not exceed the amount of the maximum compensation earnable by a person holding the position that he or she held at the time of his or her retirement, or if that position has been abolished, the maximum compensation earnable by a person holding it immediately prior to its abolition.

If his or her earnings are further altered, the board may further alter his or her disability retirement pension to the lower of the following amounts:

(a) The amount of the disability retirement pension upon which he or she was originally retired.

(b) An amount that, when added to the compensation earned by him or her shall equal the amount of the maximum compensation earnable by a person holding the position that he or she held at the time of his or her retirement, or, if that position has been abolished, the maximum compensation earnable by a person holding it immediately prior to its abolition.

The recipient of a disability allowance shall furnish earnings information as requested by the board to administer this section. If the recipient fails to furnish requested information, the disability retirement pension shall be discontinued until such time as the requested information is furnished. If the requested information is furnished, the disability retirement pension shall be reinstated.

When he or she reaches the minimum age for voluntary retirement for service applicable to members of his or her class his or her retirement allowance shall be made equal to the amount it would be if not reduced under this section, and shall not again be modified for any cause.