PLC sequence stuck on a step: permissive, feedback, and timeout troubleshooting
Diagnose a PLC sequence that will not advance by separating the active step, transition permissives, output command, device feedback, timer state, scan execution, and restart behavior.
- Product
- PLC sequence and state-machine logic
- Level
- Intermediate
- Read time
- 13 min
- Reviewed
- 2026-09-04
- Author
- Toolbox editorial team
- Reviewer
- Technical review
What to establish before troubleshooting
A sequence that appears stuck is usually waiting at one identifiable boundary: the current step is not executing, a transition permissive is false, the command is not being issued, the field device does not return feedback, or a timeout and fault path has taken ownership. Capture those states together before changing logic.
Treat the PLC program and the physical process as two synchronized stories. The step and transition logic explain what the controller expects; the command, output, device state, and feedback explain what the machine actually did. Comparing both stories at one timestamp is more useful than forcing the next step.
Restart behavior matters as much as normal advancement. Retentive step bits, first-scan logic, mode changes, paused timers, and incomplete reset conditions can return a machine to an unsafe or misleading state unless the approved recovery path is explicit and tested.
Abbreviated worked example
Example: conveyor transfer waits at the extend step
The sequence remains at Step 30 after commanding a transfer cylinder to extend. The extend output is on, but the extended limit input never becomes true and the step timer approaches its preset.
- 1Capture the active step, transition expression, extend command, output image, terminal voltage, device motion, extended-limit input, timer accumulated value, and fault bit at the same moment.
- 2Prove whether the failure is before the output, between the output and actuator, or between physical motion and returned feedback before resetting the timer or forcing the transition.
Result: The evidence separates a logic-permissive problem from an output-path, actuator, sensor, or feedback-mapping failure without bypassing the sequence.
Caution: Do not force the transition, output, or feedback bit to advance production. Place the equipment in an approved safe state and follow the site's energy-control and recovery procedures.
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