The CPU is in STOP, a red LED is lit, and the line has been down for half an hour. Read the front panel in two halves. RUN and STOP tell you what the CPU is doing. The error LEDs tell you where the fault lives: on an S7-400, INTF means inside the CPU or its program, and EXTF means out in the plant. An S7-300 collapses both into one SF LED, which is why the same fault takes longer to find on a 300.
This page covers what each LED indicates, why steady and flashing differ on the two that matter most, and which faults end in a replacement module. If you already know what failed, our SIMATIC S7 spare parts guide covers ordering.
What each LED indicates
Designations differ between families, which helps. INTF means an S7-400. SF and DC5V mean an S7-300.
| LED | Family | Steady | Flashing |
|---|---|---|---|
SF (red) |
S7-300 | Group fault pending in the diagnostic buffer. | Diagnostic interrupt from a DP station. |
BF, BF1, BF2
|
S7-300 DP or PN | Bus or interface: broken segment, termination, baud rate, no master. | A configured station is unreachable, or the network does not match the project. |
DC5V (green) |
S7-300 | 5 V to the CPU and backplane bus is present. | Not used. |
RUN (green) |
Both | CPU in RUN. | 2 Hz at startup, 0.5 Hz in HOLD. |
STOP (yellow) |
Both | CPU in STOP, HOLD or startup. | 0.5 Hz, memory reset requested. 2 Hz, reset running. |
INTF (red) |
S7-400 | Internal: CPU, user program or configuration. | See the manual. |
EXTF (red) |
S7-400 | External: a module, a wire break, a missing load voltage. | See the manual. |
BUS1F, BUS2F
|
S7-400 | Fault on that interface. | Station failure on that interface. |
REDF (red) |
S7-400H | Redundancy error or H system state. | Read the buffer. |
A slowly flashing STOP LED is a request, not a verdict
This LED costs more money than any other on the panel. A STOP LED flashing at roughly 0.5 Hz is the CPU asking for a memory reset. It is not a dead processor. It means the CPU cannot make sense of its load memory, and on an S7-300 that points at the Micro Memory Card far more often than at the CPU. Cards get pulled during a panel clean, swapped between machines and never returned, or simply reach the end of their write life. A good CPU goes in the bin with a card fault.
The check that saves an afternoon: if the CPU still asks for a memory reset after a correctly executed MRES sequence, the card is the fault. A card that cannot be initialised is at end of life. The MMC is mandatory on the 31xC, F and T variants, so keep a spare on the shelf.
Before you touch the mode selector, upload the station to your programming device. A memory reset preserves the MPI parameters and the diagnostic buffer, but clears work memory and the retentive area. If load memory turns out to be empty, you have lost the only copy of a program nobody archived since 2009. Upload first. Four minutes.
SF is a pending entry, not an operating state
An S7-300 can sit in RUN with SF lit for months. SF reflects an unacknowledged condition in the diagnostic buffer, and the CPU stayed in RUN because the matching error OB is loaded: OB82 for a diagnostic interrupt, OB86 for a station failure, OB121 and OB122 for programming and I/O access errors. Load those and the CPU tolerates faults rather than stopping.
A lit SF on a running machine means somebody has been working around a known fault, possibly for years. Read the buffer before deciding it is new. The usual causes: a DP station removed physically but never deleted from the configuration, or a channel fault inside a distributed station.
BF: steady is the bus, flashing is a station
The two states send you to different ends of the building. Steady BF is a fault at bus or interface level: a broken segment, missing or doubled termination, a baud rate mismatch between masters, or a CPU set as a DP slave with no master on the wire. Flashing BF means the CPU knows its configuration and cannot reach a station in it: switched off, wrong address, or decommissioned and left in the project. Check power and address before you reach for a cable tester.
A short BF flash that comes and goes is different. If the dropout is shorter than the configured watchdog, the CPU does not stop, but that station's outputs fall back to their substitute values meanwhile. The machine twitches and nothing is logged as a hardware failure. Look for a marginal connector, a damaged shield, or a 24 V supply sagging under load.
When the LEDs point at a module
Most of these faults end in a configuration change, a connector or a card. A minority end in hardware.
- DC5V dark on an S7-300. No 5 V from the backplane. Look at the rack supply and the bus connectors. Swapping the CPU proves nothing. We hold the SITOP 6EP1436-3BA00 20 A supply and the 6EW1811-8AA modular unit.
- BF at the station, not the CPU. Interface and connecting modules fail more often than the I/O behind them, such as the ET 200pro CM IO 8 x M12 module 6ES7194-4CB50-0AA0.
- PROFINET faults that follow a switch. Put a known good one in before condemning the CPU port: SCALANCE XB008 6GK5008-0BA10-1AB2.
- REDF on an H system. Loss of redundancy, often a sync cable or one half of a duplicated master system. We have a 6ES7414-4HM14-0AB0 CPU 414-4H and S7-REDCONNECT 6GK1716-0HB71-3AA0.
- No way to read the buffer. Classic S7-300 and S7-400 need STEP 7 V5.x, not TIA Portal. A STEP 7 V5.3 upgrade licence is often the cheapest part of the repair.
Everything we list is inspected and bench-tested where applicable, ships from EU stock, and carries a functional warranty of 14 to 30 days depending on the item. Browse the PLC CPUs collection or power supplies.
Quick answers
What does a flashing STOP LED mean on an S7-300?
At about 0.5 Hz the CPU is requesting a memory reset, meaning it cannot read a valid project from load memory. At 2 Hz a reset is already running. The slow flash is usually a Micro Memory Card that has been removed, swapped, or reached the end of its write life.
Why is the SF LED on while the CPU is still in RUN?
Because SF reports a pending diagnostic buffer entry, not the operating state. The CPU stayed in RUN because the matching error OB is loaded, typically OB82 or OB86. Something is faulty and the program is tolerating it. Read the buffer.
What is the difference between a steady and a flashing BF LED?
Steady points at the bus or interface itself: broken segment, wrong termination, baud rate mismatch, no master. Flashing points at a station that cannot be reached, or a network that no longer matches the project. Steady sends you to the cable, flashing to a device.
INTF or EXTF on an S7-400, which do I chase first?
Whichever is lit, because they split the problem for you. INTF is the CPU, the user program or the configuration. EXTF is everything outside: a signal module, a broken wire, a missing load voltage. If both are lit, work the oldest buffer entry.
Will a memory reset wipe my PLC program?
It clears work memory and the retentive area, and preserves the MPI parameters and the diagnostic buffer. If a valid project sits in load memory on the card, the CPU reloads it. If the card is empty, invalid or absent, nothing comes back. Upload first, every time.
A diagnostic order that works
- Photograph the front panel before changing anything.
- Note which error LED is lit, and whether it is steady or flashing.
- Check
DC5Von an S7-300 first. No 5 V means the rack supply. - Upload the station to your programming device, before any reset.
- Read the diagnostic buffer and work the oldest entry.
- If BF is involved, compare configured stations against what is really powered.
- Only then consider a memory reset.
- Quote the full order number and version when you source a replacement.
LED lit and a line standing still? Send us the LED pattern, the buffer entry and the order number from the label, and we will tell you what is on the shelf. Browse Siemens stock or contact us directly. Orders placed before 14:00 CET on a business day are dispatched the same day.
Related reading on Samonde
- SIMATIC S7 PLC spare parts guide
- Modicon Premium and Quantum PLC LED faults
- SINUMERIK 840D alarm codes
- Used industrial parts buying guide
Full LED tables for your order number and firmware are in the CPU manual on Siemens Industry Online Support, and the patterns shift between variants. The habit matters more than the table. Look at the panel, write down steady or flashing, upload the project, then read the buffer. Most of the S7 CPUs that reach us were never faulty, and whatever did fail went to the scrapyard with them.
