Cooling fan
Common VFD, drive, UPS, and cabinet overheating failure point.
Error Code: Siemens S7-1200 LED Status Codes
What it means: Five LEDs on the S7-1200 CPU front panel report the CPU state without a programming tool. SF (red) shows system faults. BF/Bus (yellow) shows PROFINET communication status. DC/DIAG (yellow/green) shows power supply status. RUN (green) and STOP (yellow) show the operating mode. Each LED has multiple blink patterns that map to specific conditions. This applies to all S7-1200 CPU models: 1211C, 1212C, 1214C, 1215C, and 1217C.
Common Causes
- SF flashing, CPU stays in RUN — I/O module mismatch or module failure. Most common after swapping a signal module with a different part number.
- SF solid red, CPU in STOP — Major hardware fault, cycle time exceeded, or a programming error that forces STOP.
- BF flashing yellow — No PROFINET connection. Cable unplugged, IP address mismatch, or partner device offline.
- BF solid yellow — Physical link is up but no data exchange. IO device not configured or wrong device name in the configuration.
- DC off — No 24V DC supply or supply below 20.4V. Check the L+ and M terminals with a multimeter.
- DC flashing green — CPU is providing 24V DC on the output supply. Normal behavior when outputs are enabled.
- SF and BF both flashing — PROFINET IO device fault combined with a system error. Multiple module faults.
Step-by-Step Fix
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Check the DC LED first. If DC is off, nothing else matters. Measure voltage between L+ and M on the CPU power terminals. The CPU needs 20.4 to 28.8V DC. Below 20.4V the CPU will not start reliably. Below 14V the CPU stops communicating entirely.
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If DC is solid green and RUN is off, press the RUN/STOP button to start the CPU. If the STOP LED stays solid or the CPU returns to STOP, there is a fault holding the CPU out of RUN.
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If SF is flashing red, open TIA Portal and go online. Navigate to the Diagnostics folder and read the diagnostic buffer. A flashing SF while the CPU stays in RUN means an I/O module changed or failed. Compare your online hardware configuration against the offline project. Load the correct hardware configuration to the CPU.
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If SF is solid red, the CPU is in STOP. Read the diagnostic buffer. The most common entries causing solid red SF are Event 4 (cycle time exceeded), Event 5 (I/O access error), or Event 10 (programming error). Check your OB1 and any interrupt OBs for infinite loops, blocked SCL blocks, or division by zero.
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If BF is flashing yellow, start with the PROFINET cable. Swap it with a known good cable. Check that both devices have IP addresses on the same subnet. On the CPU, go to the device properties in TIA Portal and verify the PROFINET interface settings.
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If BF is solid yellow, the cable and link are good but data is not flowing. In TIA Portal, check device names. The name assigned to the actual IO device must match the name in your hardware configuration. Use the PROFINET device name assignment tool in TIA Portal to set names on unconfigured devices.
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For combined SF and BF faults, use the pattern to narrow the diagnosis. SF flashing with BF solid means a specific IO device is missing. SF flashing with BF flashing means multiple configured devices are offline. SF solid with BF flashing means the CPU is in STOP and PROFINET communication is interrupted.
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Use the CPU web server. The S7-1200 has a built-in web server. Open a browser and enter the CPU IP address. Go to the Diagnostics page. You can read the LED states, a truncated diagnostic buffer, and module status. This feature is enabled by default on firmware 4.0 and higher. It saves a TIA Portal session for quick field checks.
LED Quick Reference Table
| LED | State | Meaning | Action |
|---|---|---|---|
| SF (red) | Off | No system fault | None required |
| SF (red) | Flashing | I/O fault or configuration mismatch, CPU in RUN | Check I/O modules, load correct configuration |
| SF (red) | Solid | Major fault, CPU in STOP | Read diagnostic buffer, fix hardware or program |
| BF/Bus (yellow) | Off | No PROFINET activity or not configured | Normal if PROFINET unused |
| BF/Bus (yellow) | Flashing | No connection on PROFINET port | Check cable, IP settings, partner device status |
| BF/Bus (yellow) | Solid | Link up, no data exchange | Check device names, IO configuration |
| DC/DIAG (yellow/green) | Off | No 24V DC supply | Check power supply, wiring, fuse |
| DC/DIAG (yellow/green) | Solid green | CPU power OK | Normal |
| DC/DIAG (yellow/green) | Flashing green | CPU providing 24V on output supply | Normal during operation |
| RUN (green) | Solid | CPU in RUN mode | Normal |
| STOP (yellow) | Solid | CPU in STOP mode | Press RUN/STOP or clear the fault |
| RUN and STOP | Alternating flash | CPU in startup or memory reset | Wait for startup to complete |
Technician Tips
The most common field call for S7-1200 LED faults is an I/O configuration mismatch after module replacement. Someone swaps a SM1231 AI 4x13bit for a SM1231 AI 8x13bit. The SF LED flashes, the CPU stays in RUN, but the analog input channel addresses shift. Always compare the catalog number on the module side against what TIA Portal expects.
For BF faults, check the PROFINET connector pins first. A bent pin 4 or 5 inside the RJ45 jack is common on industrial connectors. Carry a known good patch cable and swap it before replacing the PLC.
The web server is your best tool without TIA Portal. Every S7-1200 with firmware 4.0 or later has it enabled by default. Use the Diagnostics page to view LED states and a truncated diagnostic buffer. This can prevent a service trip just to read an error code.
Need Replacement Parts?
Finding the right replacement modules for your S7-1200 system is critical. Check current prices on Amazon. Prices and availability change frequently.
When to Call a Professional
If you have verified the 24V DC supply, tested the PROFINET cable, confirmed IP addressing, and the CPU still shows solid red SF or refuses to transition from STOP to RUN, contact a Siemens-trained technician. Share the diagnostic buffer entries and timestamps with them before arrival. This cuts their troubleshooting time in half.
If the CPU repeatedly faults with Event 4 (cycle time exceeded), do not patch the code while the machine is running. Have a Siemens automation engineer perform a full code review. A cycle time violation can mask deeper logic errors that need offline debugging.
Do not attempt to replace a CPU diagnosed with a hardware fault unless you have the original project backup. A replacement CPU needs the full project loaded, including the IP address, device name, and hardware configuration. Without the backup, you will need to re-create the entire project from scratch.
See Also
- Siemens S7-1500 Diagnostic Buffer — How to Read and Interpret Fault Codes
- Siemens S7-300 Fault Codes — Complete Diagnostic Guide
- Siemens S7-1200 CPU Fault Codes — Complete Guide