Abstract— This paper presents a one approach for creation of fault-tolerant control systems for electric servo actuators of underwater manipulators, which are describing by nonlinear differential equations with variable and unknown parameters. It is assumed, that the following faults are possible: fault due to action on the output shaft of an unknown external torque and fault, caused by error in actuator position sensor. A bank of diagnostic observers is used to provide the fault isolation and fault estimation processes. So-called logic-dynamic approach is suggested to solve this problem. The advantage of this approach is that it allows considering the systems with non-smooth nonlinearities by linear methods. The fault tolerant control is ensured due to the generation of additional control signals based on estimated value of faults. This provides the reliability of underwater working of manipulators at case of faults. The efficiency of the proposed system is confirmed by mathematical modelling.