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What kind of load (voltage, current) were these digital outputs driving? If I had to guess, I would say excessive current through the channel would have caused them to fail.
The cFP-DO-401 module may have been damaged by high voltage transients. When a channel of the [c]FP-DO-400/401 is damaged, the channel
will remain in the ON position. Though the LEDs will turn on and off
correctly, the module will always output a HIGH voltage. To prevent
against this type of failure in the future, you need to place a
properly selected transient voltage surge suppressor (TVSS) between V
and C of the module. Refer to page 11 of the manual for this caution.