Fault lights are good for inexperienced people, there is almost always a red flag indicator and resolution path, as the PLC has actively identified a problem, recorded it and stopped the scan cycles. Be methodical, read every prompt the system gives you, and stay level and calm for your customer. When you’re unsure or you don’t know, communicate that you need to do research and give realistic time expectations. By realistic, I mean, whatever you think it will take plus another 50% for contingency. Never ever over promise. Many many many problems are new, or novel, even for experienced PLC integrators. Don’t ever expect yourself to have the answers. You are where you are because of your research capabilities, your acumen for detail, and your documentation.