PLC Troubleshooting Masterclass — Fix These 7 Common Errors Today I’m going to show you the *7 most common PLC programming mistakes* that cause your logic to fail… and more importantly, *how to fix each one . If your PLC program is not working, 90% of the time one of these mistakes is the reason. Let’s start!” #1: Wrong NO/NC Contact Selection* common beginner error)* “The first and most common mistake — using a Normally Open contact instead of Normally Closed, or vice versa.” *Example scenario: