## 3 Case Study: Structural Factor Analysis for Public Transportation Bus
### Result Analysis
#### 3.5.1 Cause Group Analysis  
The factors of the cause group have net impacts on the overall vehicle in a possible accident event. Therefore, such factors should be paid more attention by engineers in maintenance management. In our numerical study, out of the 20 critical factors, 18 factors are identified as belonging to the cause group. The components involved in those factors are the transmission cable (CF3), lubricating oil (CF6), ignition switch sensor (CF7), engine speed sensor (CF8), diesel injector (CF9), engine cylinder (CF10), turbocharger (CF11), EBS sensor wire (CF14), EBS valve (CF15), brake system air hose (CF16), air brake dryer (CF18), balance lever (CF21), shock absorber (CF22), battery (CF26), alternator (CF27), alternator belt components (CF28), electrical appliance central control system (CF29), and starter motor (CF30). In the cause group, the ignition switch sensor (CF7) of the vehicle has the highest value of \((D-R)\), which means that the ignition switch sensor (CF7) has more of an impact on the whole system failure. In addition, Table 5 indicates that the ignition switch sensor (CF7) holds the second rank position based on the \((D+R)\) value, which suggests that the ignition switch sensor (CF7) is a significant factor in vehicle failures. An ignition switch sensor is a vital component, responsible for detecting the position of the ignition switch and communicating this information to the electronic control units. This sensor  
Figure 2: Influence diagram generated after IVPF-DEMATEL calculation. plays an important role in ignition by ensuring that the engine starts only when the correct key or signal is detected. It ensures that the engine starts reliably and all necessary systems are powered up in the correct sequence, which is critical for maintenance activity schedules and operation safety. Common failure types of ignition switch sensors include intermittent starting issues, failure to start, and erratic engine behavior. Such malfunctions can lead to significant operational issues, such as service delays, increased maintenance costs, and compromised passenger safety. Additionally, a faulty ignition switch sensor can trigger unforeseen expansion events, such as engine problems or power distribution problems. This propagation mechanism typically begins with deteriorating sensor performance, which leads to inconsistent signal transmission to the Engine Control Unit (ECU). Compromised ECU functionality subsequently affects various engine management parameters, including fuel injection timing, ignition timing, and air-fuel mixture ratios. These disruptions can manifest in multiple ways, such as irregular engine performance, increased fuel consumption, and potential damage to catalytic converters. Furthermore, the interdependencies between electrical and mechanical systems amplify the impact of maintenance-related failures. when the ignition switch sensor exhibits intermittent failures due to delayed maintenance, which can generate voltage fluctuations throughout the vehicle's electrical network. These electrical disturbances can affect multiple electronic control modules, potentially compromising the functionality of safety-critical systems such as anti-lock braking systems (ABS), electronic stability control (ESC), and transmission control units. The cumulative effect of these cascading failures not only increases the complexity of diagnostic procedures but also significantly elevates repair costs and vehicle downtime. For similar reasons, the battery (CF26), alternator (CF27), turbocharger (CF11), and starter motor (CF30) can be regarded as vital factors, resulting in public transportation failure events, and should be prioritized in maintenance management. The dual-ranking underscores the need for targeted maintenance strategies and resource allocation to these high-risk and high-impact factors.