This paper investigates the fault detection problem of quantized feedback systems with actuator faults, and system outputs are respectively quantized by a uniform quantizer and a non-uniform quantizer. A new method is proposed to deal with quantization errors, that is, the quantization errors are considered as the unknown inputs of the residual generator. In order to detect the faults, an optimal residual generator is first designed to guarantee a suitable compromise between the sensitivity to faults and the robustness against disturbances as well as quantization errors, and then the residual signal is evaluated; finally, two different thresholds on the basis of the stochastic properties of quantization errors are designed for the two types of quantizers. A simulated example is given to illustrate the effectiveness of the proposed method.