The short answer
On S7-1200 and S7-1500 CPUs, the RUN/STOP LED shows the mode – green for RUN, yellow for STOP – and a flashing red ERROR LED means the CPU has found an error, such as a program, memory card or configuration problem, or a diagnostic event on a module. A yellow MAINT LED means maintenance is pending, or on an S7-1500 that a force job is active; on an S7-1200 a steady red ERROR LED means defective hardware. The CPU's diagnostic buffer – in TIA Portal under Online & diagnostics, on the S7-1500 display or in the web server – tells you what actually happened.
This covers Siemens SIMATIC S7-1200 CPUs (CPU 1211C to 1215C, 6ES7 21x) and S7-1500 CPUs (6ES7 51x) programmed with STEP 7 in TIA Portal. For the older S7-300 and S7-400, see SF and BF LEDs and the STEP 7 diagnostic buffer.
Key takeaways
- ERROR flashing red: an error or a pending diagnostic event. Read the diagnostic buffer before you reset or swap anything.
- A steady yellow MAINT LED on an S7-1500 can mean an active force job, not only a maintenance alarm.
- A memory reset keeps the diagnostic buffer on both families; a reset to factory settings deletes it.
The S7-1200 CPU LEDs
| STOP/RUN · ERROR · MAINT | What it means (our words) |
|---|---|
| All off | The CPU has no power. |
| Yellow and green alternating | Startup, self-test or a firmware update in progress. |
| Yellow / green | STOP / RUN. |
| Yellow, MAINT flashing | A memory card was inserted; the CPU went to STOP and waits. |
| ERROR flashing red | An error: inside the CPU, on the memory card, or modules that don't match the configuration. |
| MAINT steady | Maintenance has been requested. |
| Yellow, ERROR steady red | Defective hardware. |
| All three flashing | The LED test, or defective CPU firmware. |
The S7-1200 LED meanings here follow Siemens' S7-1200 system manual edition 04/2012 (firmware V3). Newer CPU firmware may add states; check the system manual edition for your firmware.
After you insert a memory card, the S7-1200 only reads it when you switch the CPU to RUN, do a memory reset (MRES) or cycle its power. Each signal module also has a DIAG LED: green when it is working, red when it is defective or not operating; on an analog module, a red channel LED marks an error on that input or output.
The S7-1500 CPU LEDs
| RUN/STOP · ERROR · MAINT | What it means (our words) |
|---|---|
| All off | The CPU's supply voltage is missing or too low. |
| Green / yellow | RUN / STOP. |
| RUN/STOP off, ERROR flashing | An error has occurred. |
| Green, ERROR flashing | Running, with a diagnostic event pending, such as a fault on a module. |
| Green, MAINT steady yellow | Maintenance demanded (check or replace hardware soon), or a force job is active. |
| Green, MAINT flashing | Maintenance required within a foreseeable time, or a bad configuration. |
| Yellow, ERROR and MAINT flashing | An error state: for example a fault in the program held on the SIMATIC memory card, or a defective CPU. |
| RUN/STOP flashing yellow | Busy in STOP, such as loading the program from the memory card. |
| Yellow/green flashing | Starting up; with ERROR and MAINT also flashing, booting or the LED test. |
Some newer CPUs, such as the CPU 1511C-1 PN, add a STOP ACTIVE LED: lit yellow means the CPU was stopped with its own STOP button, and then only its RUN button – not the display or TIA Portal – can put it back into RUN. S7-1500 I/O modules have their own RUN and ERROR LEDs; see I/O module LEDs.
How to read the diagnostic buffer in TIA Portal
- Select the CPU in the project tree and click Go online.
- Open the CPU's Online & diagnostics and select Diagnostics > Diagnostics buffer.
- The newest event is at the top. Entries include module errors, CPU system errors, program errors, mode changes such as RUN to STOP, and modules pulled or plugged.
- Select an entry for Details on event. Open in editor takes you to the module in the device view, or opens the block offline at the spot where the fault came from.
- In the online device view, a diagnostic symbol marks each faulty module; double-click it for that module's diagnostic status.
The Online tools task card mirrors the RUN/STOP, ERROR and MAINT LEDs and shows cycle time and memory use. On an S7-1200, the buffer holds a limited number of events that depends on the CPU, and only the most recent survive a power cycle – so read it before you switch off.
On the S7-1500 display and the web server
- Display: the Diagnostics menu shows the diagnostic buffer and pending alarms; the Modules menu shows the status of each local module, slot by slot.
- Web server: once it is enabled in the CPU's properties in TIA Portal, browse to the CPU's IP address. Its standard pages include the diagnostic buffer and module information, and on the S7-1500 also alarms and the network topology – no engineering software needed.
- Service data: if Siemens support asks for it, an S7-1500's service data can be saved through the web server, STEP 7 or the memory card.
First checks
- Read and save the diagnostic buffer before any reset, power cycle or module swap.
- All LEDs off: have a qualified electrician check the supply at the CPU's power terminals against the rating on its label. That is live work – AC-powered S7-1200 CPUs take 120/230 V AC – under your site's live-work and arc-flash rules. On an S7-1500 rack, also check the system power supply.
- ERROR flashing with RUN green: find the module with the diagnostic symbol and read its diagnostics, for example a wire break or missing supply voltage.
- Configuration errors (mismatched modules on the S7-1200, bad configuration on the S7-1500): compare the rack with the device configuration, module by module – order number and firmware version.
- MAINT steady on an S7-1500: check for an active force job before anyone touches the machine.
- Memory card problems: on an S7-1500 the load memory – your program – is on the SIMATIC memory card, so keep that card when you swap the CPU. Replacing the CPU? Match the order number and firmware version – see reading Siemens order numbers.
Safety first: only qualified personnel should work in the cabinet, and any measurement on energized equipment follows your site's live-work and arc-flash rules. Switch off and lock out the station's supply and any separately powered I/O before you pull or plug modules, connectors or the memory card, 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.
Parts that commonly fail
CPUs, I/O modules and memory cards are the hardware behind many of these LEDs. The order numbers we have the most of in stock right now:
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 S7-1200 Programmable controller System Manual, manual A5E02486680-06, 04/2012, entry 36932465.
- Siemens, SIMATIC S7-1500 / ET 200MP Automation System System Manual, manual A5E03461182-AC, 12/2014, entry 59191792.
- Siemens, SIMATIC S7-1500 CPU 1511-1 PN (6ES7511-1AK00-0AB0) Manual, manual A5E31264526-AC, 12/2014, entry 68020492.
- Siemens, SIMATIC S7-1500 CPU 1511C-1 PN (6ES7511-1CL03-0AB0) Equipment Manual, manual A5E40898565-AC, 05/2024, entry 109963074.
- Siemens, System Diagnostics with S7-1500 and TIA Portal, application example V2.0, 09/2020, entry 68011497.
Frequently asked questions
What does a flashing red ERROR LED mean on an S7-1200?
Siemens describes it as an error: for example an internal error in the CPU, a problem with the memory card or a configuration error such as modules that don't match the configuration. A steady red ERROR LED with a yellow STOP/RUN LED means defective hardware. Go online in TIA Portal and read the diagnostic buffer under Online & diagnostics to see which one it is.
Why is the MAINT LED on my S7-1500 lit yellow?
With a green RUN/STOP LED, a steady yellow MAINT LED means maintenance is demanded – hardware to check or replace soon – or a force job is active. A flashing MAINT LED means maintenance is required within a foreseeable time, or a bad configuration. The diagnostic buffer and the module's diagnostics say which.
Where is the diagnostic buffer in TIA Portal?
Go online with the CPU, open its Online & diagnostics from the project tree and select Diagnostics > Diagnostics buffer. The newest event is at the top; select one for the details, and Open in editor takes you to the module or the program block involved. On an S7-1500 you can also read it on the CPU display (Diagnostics menu) or on the web server's Diagnostic buffer page.
Does a memory reset clear the diagnostic buffer?
No, on either family. An S7-1500 memory reset keeps the diagnostic buffer, the time of day and the IP address; an S7-1200 memory reset keeps the buffer and the saved IP address. A reset to factory settings deletes the entries on both. Save or note the entries you need before any reset.