S7-300 and S7-400 CPU STOP diagnostics
Diagnose a classic SIMATIC CPU STOP by preserving the diagnostic-buffer sequence and separating programming, cycle-time, I/O access, rack, module, communication, memory, and hardware events.
- Product
- SIMATIC S7-300 and S7-400 CPUs
- Level
- Advanced
- Read time
- 16 min
- Reviewed
- 2026-08-13
- Author
- Toolbox editorial team
- Reviewer
- Technical review
What to establish before troubleshooting
The newest diagnostic-buffer entry is not always the initiating event; read the sequence around the RUN-to-STOP transition and correlate the requested organization block, address, rack, or module.
Preserve the buffer, CPU and module LEDs, hardware diagnostics, project state, time, and recent changes before warm restart, memory reset, or replacement.
Abbreviated worked example
Example: CPU STOP follows an area-length error
An S7-300 CPU enters STOP after an intermittent analog module issue, and the buffer contains several follow-on events.
- 1Save the complete diagnostic buffer and identify the first incoming event before the mode transition.
- 2Map the reported logical address to hardware and the faulting block before restarting.
Result: The initiating access or module boundary is proven instead of treating every later event as a separate cause.
Caution: Loading an empty error OB simply to keep a CPU in RUN can mask a serious fault; design diagnostic OB behavior and machine response deliberately.
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