{ "background": "Maintenance systems for public transport depots in developing economies are often characterised by inefficiency and high operational costs. There is a paucity of rigorous, field-based evidence on the cost-effectiveness of different diagnostic and intervention protocols within these engineering management systems.", "purpose and objectives": "This case study aimed to methodologically evaluate and compare the cost-effectiveness of a proposed structured diagnostic protocol against prevailing practice in depot maintenance. The primary objective was to quantify the difference in mean cost per effective repair.", "methodology": "A randomised field trial was conducted across multiple transport depots. Depots were randomly assigned to either the intervention group, implementing a new diagnostic protocol, or a control group continuing with existing practices. Cost and outcome data were collected over a standardised period. Cost-effectiveness was analysed using a generalised linear model: $Ci = \\beta0 + \\beta1 Ti + \\beta' Xi + \\epsiloni$, where $Ci$ is the log-transformed cost per effective repair for depot $i$, $Ti$ is the treatment indicator, and $X_i$ is a vector of covariates. Inference was based on robust standard errors.", "findings": "Implementation of the structured diagnostic protocol yielded a statistically significant reduction in mean cost per effective repair. The point estimate indicates a cost reduction of approximately 22% (95% confidence interval: 15% to 28%) compared to the control group, after controlling for depot size and vehicle fleet age.", "conclusion": "The randomised trial demonstrates that a structured, evidence-based diagnostic protocol can substantially improve the cost-effectiveness of maintenance operations in this context.", "recommendations": "Transport authorities should consider adopting standardised diagnostic frameworks informed by operational data. Further research should investigate the scalability of this approach and its integration with digital maintenance management systems.", "key words": "maintenance management, randomised controlled trial, cost-effectiveness analysis, transport engineering, infrastructure management", "contribution statement": "This study provides the first application