The reliability and overall cost of unattended engine cabin are researched for two aspects: fault diagnosis and preventive maintenance in this paper. The ME-OWA algorithm is adopted to predict the fault diagnosis of per module in the unattended engine cabin and the cost of preventive maintenance is calculated, its costs are controlled by adopting the optimal number of preventive maintenances under the object of minimum reliability. The optimization of maintenance cost can be achieved by repairing modules with excessive fault diagnosis rates under the determining preventive maintenance times and reliability. The reliability model of the unattended engine cabin is founded to the series-parallel structure of per module, and its reliability is compared between the regular maintenance and the irregular maintenance. The calculations show that the reliability of the unattended engine cabin is more stable under the irregular maintenance.