Process failures are instigated by underlying errors and faults in various layers of WSN (Wireless Sensor Network) communication protocol stack. Therefore, efficient and effective monitoring systems for fault detection and diagnosis are imperative for fault tolerance and robust operation of WSN to meet critical application requirements for reliability and throughput. Existing detection-diagnosis regimen are either centralized or distributed and network monitoring is performed passively or actively. This work presents a diagnostic agent based inter-process communication aware monitoring system for WSNs. Diagnostic agent actively performs probe-based process execution tracking and examines the effects of errors, omissions and channel misbehavior on process execution at node, link and network levels to implement failure detection and fault diagnosis. Such diagnosis is performed through the inference of inter-process communication of stacked and peer layer processes on sender and receiver side. The monitoring system has been implemented in Castalia simulator for WSN. Local diagnostic agent is implemented on sensor nodes for self-monitoring and network wide fault diagnosis is performed by global diagnostic agent on cluster head. Simulation results show that the system performs robust root cause analysis of critical process failures due to errors in stacked and peer layer processes. The decentralized distribution of diagnostic load on sensor nodes and cluster head produces lesser communication overhead and is energy efficient.