Most of the wireless sensor networks are energy limited. To improve the lifetime of a network, the energy consumption should be reduced. Clustering is a proven technique that reduces energy consumption in WSNs. The energy can be saved by sending the collected information to the sink using the Cluster Head (CH) nodes. Fault tolerance is a crucial issue in WSNs, and one cluster head failure collapses the entire data communication. This paper presents a fault tolerance cluster-based routing technique. This paper aims to provide improved fault tolerance and data aggregation technique for clustered WSN using FTOR-ModPSO (Fault tolerance and optimal relay node with modified Particle Swarm Optimization). Here, the fault tolerance is achieved by selecting the backup cluster head (BKCH) for every cluster. The method consists of two phases. The network is categorized into clusters in the first phase whereas in the second phase, CHs and BKCHs are selected. Intra-cluster communication happens between member and CH nodes. Aggregator nodes (AG) are used for inter-cluster communication, and modified PSO identifies CH and AG's best relay. The fault tolerance and packet delivery rate are increased with less energy utilization compared to other existing energy-saving protocols by implementing this method.