This paper investigates the path tracking problem for an electric vehicle (EV) which has four electromechanical wheel systems under normal and faulty conditions. With considering wheel slip constraints and certain actuator faults, a fault-tolerant controller with observer-based fault diagnosis is designed to maintain the system stability and guarantee the tracking performance. Then based on the designed controller, an active fault diagnosis approach is introduced for this typical over-actuated system to isolate and evaluate faults more precisely. Finally, simulations of traction engine faults in RobuCar TM situation are conducted to test the feasibility of the proposed method.