The reliability and performance of public transport systems are fundamental to ensure smooth operations and minimize disruptions. This study presents a novel analysis of maintenance activities and their impact on both intrinsic and operational availability within a municipal bus fleet, using a unique dataset from 53 Autosan Sancity M12LF buses. The study investigates Key Performance Indicators such as Mean Time Between Failures (MTBF), Mean Time to Repair (MTTR), and Availability, along with the impact of a planned maintenance schedule. The objective is to assess how maintenance practices affect operational efficiency and fleet performance, as well as to recommend new approaches aiming to increase the operational Availability. The results revealed that the fleet had an average MTTR of 2.66 h and an MTBF of 26.55 h, yielding 90.69% Intrinsic Availability. Operational Availability was lower, at 78.47%, due to maintenance interventions. A high proportion of unplanned maintenance (92.23%) suggests a need for strategies to reduce failures. The analysis revealed a moderate negative correlation between vehicle age and reliability and indicated that unplanned maintenance significantly reduced operational efficiency. These findings suggest that improving preventive maintenance strategies can enhance overall fleet performance.