The paper presents the sensorless five phase induction motor drive implementation with an inverter output filter with third harmonic injection. The used observers are presented as well as a control system that is able to handle the rotor flux synchronization of the first and third harmonic. Moreover, the applied control systems with the presented observers are capable to handle a deactivation of one or two phases which improves the reliability of the complete drive system. A new idea of online open phase fault detection is presented based on the frequency analysis of the estimated observer variables. The test setup, fault operation capabilities as well as an experimental verification of the proposed fault detection method are presented in the paper.