A method and apparatus for detecting faults in an injector arrangement is described. The injector arrangement comprises a plurality of piezoelectric injectors that are located in parallel branches of an injector bank circuit of an injector drive circuit. Each branch of the injector bank circuit comprises a high side isolation switch. The high side isolation switches are each operable to enable an associated piezoelectric injector in the injector bank circuit when closed, and disable the associated piezoelectric injector in the injector bank circuit when open. The fault detection method comprises the steps of operating the high side isolation switches so as to enable one of the piezoelectric injectors and disable the other piezoelectric injector(s), and performing diagnostics to detect the presence or absence of faults on the enabled piezoelectric injector.