The two main causes of packet corruption in wireless sensor network are multipath fading and WiFi interference. Identifying the cause of the corruption can be used to improve the reliability of the transmission. If the corruption is caused by WiFi interference, the network could change the channel to a free one. If it is caused by multipath fading, the network could reroute the traffic away from the obstacles or shorten the distance. This project proposes a new method of corruption-causes-identification for the two causes mentioned. It is an immediate online diagnostic tool for IEEE 802.15.4 packets with a retransmission mechanism. It provides a statistical boundary with a processed deviation of RSSI value and the frame symbol error rate, and also a rechecking mechanism to check the decisions. In this model, the deviation of RSSI value is measured by the estimated RSSI value of correct packets and the current detected RSSI value. The benefit of the deviation design is that the statistical model hardly needs to retrain and correct the parameters in different transmission environments. The project also discusses two rechecking mechanism methods, one employs an individual secondary classification with its own characters; the other combines the neighbor packets' features to smooththe probable errors. To investigate the performance of the "frame symbol errorrate and deviation of RSSI values"-based diagnostic tool, the evaluation parts provide a comparison of different length packets. The conclusion is that this diagnostic tool provides a correct judgment of the accuracy of packet corruption caused by multipath fading up to 98.70%, and an accuracy of up to 92.99% for the WiFi-interferenced packet corruption.