The S7-1200 logs error code 401 in the diagnostic buffer and the connected HMI or partner stops exchanging data, while the CPU RUN light may still be on.
These are the failure paths we see most often on the Siemens S7-1200 for a error code 401 condition. Likelihood reflects field frequency; First Check is what you can do remotely before sharing logs.
| Cause | Likelihood | First Check |
|---|---|---|
| PROFINET/communication partner not reachable (IP or name mismatch) | High | Compare the device's configured name/IP to the actual partner and the station config |
| Hardware configuration mismatch after a download | Medium | Check the diagnostic buffer for 'component not found' and compare the offline/online config |
| Flying restart / watchdog on a specific OB not loaded | Medium | Look for a missing OB (e.g. OB86/OB122) that the fault references |
| Address conflict with another node on the network | Low–Medium | Scan the subnet for a duplicate IP or device name |
Work through these in order. Each step is something you can perform and report back on without opening the enclosure or swapping parts.
In TIA Portal or via the web server, read the diagnostic buffer and capture the 401 entry with its timestamp and any referenced OB or station. The detail text is the key.
Confirm the communication partner's IP and device name match what the CPU expects. A renamed device or a changed IP is the usual trigger for a 401.
Compare the downloaded hardware configuration to the actual module. A missing or different module logs a config fault that surfaces as 401.
If the fault references an organization block you have not loaded, add that OB (even empty) so the CPU handles the event instead of faulting. Note which OB the buffer names.
After correcting the name/IP or adding the OB, go online and confirm the 401 clears and data exchange resumes with the partner.
A 401 on the S7-1200 is a configuration or communication fault, almost never a dead CPU. The diagnostic buffer entry — especially any referenced OB or station — tells us precisely where the mismatch is. Cross-reference the broader collections: Error code / fault buffer · PLC & PROFINET.
Video ID is a placeholder. Replace REPLACE_WITH_REAL_ID with the real YouTube video ID before publishing; the facade loads youtube-nocookie only after a click.
Almost never. A 401 is a config or link mismatch we resolve from the buffer. We only suspect hardware if the buffer shows a physical fault, and we confirm that.
It is the pattern where a brief loss of a partner trips a fault that an unloaded OB would have absorbed. We identify the missing OB from the buffer and tell you which to add.
No. You read the buffer via TIA Portal or the web server and relay it; we interpret it. We do not take control of your controller.
Send the diagnostic buffer entry and the partner config. We pinpoint the name/IP or missing-OB fault remotely so you avoid swapping good hardware.
Submit Your Case Connect With SpecialistsWe do not perform hardware repair, we do not replace parts, and we cannot promise a 100% resolution. Our role is limited to remote diagnostic and step-by-step guidance based on the information, photos and logs you share.
When the 401 is really a broader link loss.
The partner side that often shows the 401 first.
Full machinery controller troubleshooting collection.