For one of my (simplest) problems, please see the attached pdf. (I re-typed some stuff in RSLogix500 so I could have some comments and cross-referencing.) For some reason the hydraulic pump motor starter output won't latch in, even though I can see the input light from the pressure switch come on. Is is possible for the program to get corrupted such that the processor or memory lights do not illuminate, but yet something this simple won't work?
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One of my most unusual troubleshooting problems (EVER) was the exact thing you are describing. The I/O lights for the input were right..on the input module BUT, the program did not turn on the output. We used high speed video cameras, voltage recorders..the works..nothing made sense.
In your case, as in mine, there's corruption, alright. Programs don't just start not working.
In my case, the cause was a faulty chassis (aka rack) that held the I/O cards. We replaced it, and all our problems went away. Try changing out the I/O chassis, and see what happens.