Micro-electrode array (MEA) is widely used in bio-sensor systems. The surface degradation of MEA is a frequently reported problem by its end-users (Qinetiq Winfrith). A self-testing method is developed by comparing the impedance of each electrode and thus obtaining a distribution of all the impedance amplitude comparing with an arbitrary one in a 2D plane. Based on the fact that the surface degradation will generally increase the impedance and the assumption that the majority of the electrodes are fault-free, then the degraded electrodes can be identified. In the following parts of this paper, firstly analysis of electrode degradation based on its interfacial impedance is given with the support of both simulation and experiment data. Then the developed self-testing idea is explained in detail. A conclusion is drawn finally. This work continues from a preliminary early work published in IMSTW'6 [l].