The short answer
On an S7-300 CPU, a red SF (system fault) LED is a group error: a hardware fault or a program or parameter error somewhere in the station, and the CPU may still be in RUN. A red BF (bus fault) LED points at a PROFIBUS DP or PROFINET interface: steady usually means a cable or interface problem, flashing means a configured station isn't answering. S7-400 CPUs use INTF and EXTF instead of SF, and BUS1F, BUS2F and BUS5F for the interfaces. The CPU's diagnostic buffer says what actually happened.
This covers Siemens SIMATIC S7-300 (6ES7 31x) and S7-400 (6ES7 41x) CPUs programmed with STEP 7. See also reading Siemens order numbers.
Key takeaways
- S7-300: SF covers hardware and software errors. S7-400: INTF (internal) and EXTF (external).
- BF steady: the bus or interface itself. BF flashing: a station or IO device that doesn't answer.
- Read and save the diagnostic buffer before you power-cycle, reset memory or swap a module.
The S7-300 CPU LEDs
| LED | Color | What it means (our words) |
|---|---|---|
SF | Red | A hardware fault or software error – one group indicator for the station. |
BF, BF1–BF3 | Red | A bus fault at interface X1, X2 or X3 (PROFIBUS DP or PROFINET, by CPU). |
MAINT | Yellow | A maintenance request is pending (PROFINET CPUs). |
DC5V | Green | Lit when the CPU's internal 5 V supply is present. |
FRCE | Yellow | A force job is active; flashing at 2 Hz is the node flash test. |
RUN | Green | RUN; flashes during startup. |
STOP | Yellow | STOP, HOLD or startup; slow flashing asks for a memory reset. |
SF with STOP means the CPU stopped because of an error. If SF, DC5V, FRCE, RUN and STOP all flash, the CPU has an internal system error: Siemens says to set the mode switch to STOP, cycle power, read the diagnostic buffer and contact Siemens.
S7-400: INTF, EXTF and BUSxF
- INTF: an internal error, such as a programming or parameter error or a cycle-time overrun.
- EXTF: the cause is outside the CPU – a module diagnostic interrupt, a module pulled or inserted, a failed rack, DP segment or PROFINET IO system, an I/O access error or a backup battery problem.
- BUS1F / BUS2F / BUS5F: the MPI/DP, PROFIBUS DP and PROFINET interfaces. Lit: an error at that interface. Flashing: one or more devices on it aren't responding.
- Power supply: on PS 405/407 supplies, red BAF means the backup voltage is too low, yellow BATTF or BATT1F/BATT2F a battery that is empty, missing or reversed – when the BATT.INDIC switch is set to monitor it.
Common causes of SF (or INTF/EXTF)
- A missing error OB. For most events, if the OB the event calls isn't loaded, the CPU goes to STOP: OB 85 (access to a missing or faulty module during the process image update), OB 121 (programming error), OB 122 (I/O access error), OB 86 (failed DP slave, rack or IO device), OB 82 (diagnostic interrupt), OB 80 (cycle time exceeded).
- A missing or faulty module, one removed or inserted in RUN, or a loose connector.
- Configuration mismatch: HW Config doesn't match the rack. Diagnose Hardware marks a module that is missing or of a different type.
- A faulty memory card: an S7-300 CPU goes to STOP and asks for a memory reset.
- Backup battery (S7-400): a discharged battery lights EXTF; the CPU stays in RUN even without OB 81. S7-300 CPUs with a Micro Memory Card need no backup battery.
BF steady or flashing
- PROFIBUS, steady: a broken or shorted cable, a DP interface fault, or different transmission rates on one bus.
- PROFIBUS, flashing (CPU as master): a station failed, a configured slave can't be reached, or the configuration is wrong. By default a master CPU still starts with a slave missing, BF flashing.
- PROFINET, steady: no link to the subnet or switch, wrong speed or no full duplex; Siemens' checklist includes a switch rather than a hub, at 100 Mbit/s full duplex.
- PROFINET, flashing: one or more assigned IO devices have failed or can't be addressed.
How to read the diagnostic buffer in STEP 7
- In SIMATIC Manager, open the project online or find the CPU under Accessible Nodes.
- Select the CPU, choose PLC > Diagnostics/Settings > Module Information and open the Diagnostic Buffer tab.
- Entry 1 is the newest. Times come from the CPU clock, so set it.
- Select an entry for the event ID, the mode change it caused, the block and address involved and whether the event is coming or going; Help on Event explains it.
- If the CPU stopped because an error OB was not loaded, the entry before is the real error; Open Block and the Stacks tab show where.
- Save As keeps the buffer as a text file.
The buffer can't be cleared and survives a memory reset; when full, the oldest entry drops out. S7-300 CPUs with a Micro Memory Card keep only the last 100 entries through a power cycle. PLC > Diagnostics/Settings > Diagnose Hardware lists every module the CPU reports as faulty.
First checks
- Read and save the diagnostic buffer.
- Check DC5V and the power supply, then each module's own LEDs.
- For BF, walk the bus: connectors seated, no breaks or shorts, correct addresses, every slave or IO device powered.
- Compare the rack with HW Config: order number and slot of every module.
- On an S7-400, check BAF and BATTF on the power supply.
- Replacing a module? Match order number and product version; see S7-300 lifecycle status if it is no longer sold new.
Safety first: only qualified personnel should work in the cabinet. Switch off and lock out the rack's supply and any separately powered I/O before you pull or plug modules or bus connectors, following Siemens' instructions and your site's lockout/tagout procedure. Never force outputs, jumper interlocks or bypass a safety circuit to get a station running.
Need program changes instead? Our engineering team does PLC programming on Siemens systems too.
Parts that commonly fail
CPUs, signal modules, PROFIBUS connectors and interface modules, power supplies and memory cards are the hardware behind many of these LEDs. The order numbers we have the most of in stock right now:
S7-300 CPUs
- 6ES7315-1AF03-0AB0Siemens 6ES7315-1AF03-0AB0 Simatic S7-300 CPU 315 PLC 24 VDC 0.…
- 6ES7 315-1AF03-0AB0Siemens Simatic S7-300 6ES7 315-1AF03-0AB0 CPU 315 Central Proc…
- 6ES7314-6CG03-0AB0Siemens Simatic S7-300 6ES7314-6CG03-0AB0 CPU 314C-2 DP Compact…
- 6ES7315-2AH14-0AB0Siemens 6ES7315-2AH14-0AB0 Simatic S7-300 CPU 315-2 DP 24VDC 25…
S7-300 and S7-400 signal modules
- 6ES7421-1BL01-0AA0Siemens 6ES7421-1BL01-0AA0 Simatic S7-400 Digital Input Module …
- 6ES7322-1BH01-0AA0Siemens 6ES7322-1BH01-0AA0 Simatic S7-300 SM322 16 DO 24VDC 0.5…
- 6ES7322-1BL00-0AA0Siemens Simatic S7-300 6ES7322-1BL00-0AA0 SM322 Digital Output …
- 6ES7322-1HF01-0AA0Siemens 6ES7322-1HF01-0AA0 Simatic S7-300 SM322 8 Relay Digital…
PROFIBUS connectors, ET 200M interfaces and CPs
- 6ES7153-1AA03-0XB0Siemens 6ES7153-1AA03-0XB0 ET 200M IM 153-1 Interface Module Pr…
- 6ES7153-2BA02-0XB0Siemens 6ES7153-2BA02-0XB0 Simatic S7 ET 200M IM 153-2 Interfac…
- 6ES7153-2AA02-0XB0Siemens 6ES7153-2AA02-0XB0 Simatic S7 ET 200M IM 153-2 Interfac…
- 6GK7343-5FA01-0XE0Siemens 6GK7343-5FA01-0XE0 Simatic Net CP 343-5 Profibus Commun…
S7-300 and S7-400 power supplies
Stock changes every day. Every listing is photographed, so check the catalog number, series or product version and firmware on the label photo before you buy.
Not listed, or need several? Send us the part number and we'll check our shelves and our sourcing network and quote it in writing.
Sources
Every code, LED state and menu path on this page was checked against Siemens' own documents, listed below, and is described here in our own words. Manuals change between editions and firmware revisions: for your exact catalog or order number, check the current edition.
- Siemens, SIMATIC S7-300 CPU 31xC and CPU 31x: Technical specifications, manual A5E00105475-12, 03/2011, entry 12996906.
- Siemens, SIMATIC S7-300 CPU 31xC and CPU 31x: Installation, operating instructions A5E00105492-12, 03/2011, entry 13008499.
- Siemens, S7-400 Automation System, CPU Specifications, manual A5E00850746-10, 08/2011, entry 53385241.
- Siemens, S7-400 Automation System Module Data, reference manual A5E00850736-08, 11/2016, entry 1117740.
- Siemens, Programming with STEP 7 V5.5, manual A5E02789666-01, 05/2010, entry 45531107.
Frequently asked questions
What does a red SF LED mean on an S7-300 CPU?
SF is the S7-300's group fault indicator: Siemens describes it as a hardware fault or a software error, and internal and external faults share the one LED. The CPU can still be in RUN while it is lit. Read the CPU's diagnostic buffer to find which module or which program error set it.
What is the difference between BF lit and BF flashing?
On a PROFIBUS DP interface, a steady BF points to the bus or the interface itself: for example a broken or shorted cable, or transmission rates that don't match. A flashing BF means the bus is up but at least one configured station isn't answering, or the configuration is wrong. On a PROFINET interface, steady means no usable link to the network and flashing means one or more IO devices can't be reached.
Does an S7-400 CPU have an SF LED?
No. S7-400 CPUs split it in two: INTF for internal errors such as programming or parameter errors, and EXTF for errors whose cause is outside the CPU module, such as a module diagnostic interrupt, a rack or station failure or a discharged backup battery. Bus faults show on BUS1F, BUS2F and, on PN CPUs, BUS5F.
Where do I find the diagnostic buffer in STEP 7?
In SIMATIC Manager, go online to the CPU, choose PLC > Diagnostics/Settings > Module Information and open the Diagnostic Buffer tab. Select an entry to see its event ID, the block and address involved and whether the event is coming or going; Help on Event explains it, and Save As writes the buffer to a text file.