Hey there! I know exactly what you're talking about - those frustrating 'ghost in the machine' PLC faults that vanish the moment you arrive on site. It's like the equipment knows you're coming! Here's the systematic approach that separates veterans from rookies:

First, don't just show up and start poking around. Start with comprehensive documentation review - electrical schematics, PLC programs with comments, I/O address lists, network topology, and especially maintenance history. Look for patterns in previous failures that junior techs might miss.

Second, implement systematic data logging. Set up trend logging on suspect I/O points to capture what's happening when you're not there. Environmental factors (temperature, vibration, humidity) often trigger intermittent faults at specific times.

Third, check for 'invisible' issues: ground loops, power supply fluctuations, communication noise, and multiplexing failures in I/O racks. These often don't show up on basic diagnostics.

Fourth, look at timing and sequencing issues. Use the PLC's diagnostic tools to monitor scan times and program execution. Sometimes the problem isn't what's happening, but when it's happening.

Finally, create a 'fault reproduction' checklist. Document exactly what was happening when the fault occurred - specific machine states, operator actions, environmental conditions. This helps you recreate the scenario systematically.

The key difference? Veterans don't just fix what's broken - they understand why it broke and when it breaks. They look for patterns, not just problems.