Cooling fan
Common VFD, drive, UPS, and cabinet overheating failure point.
Siemens S7-300 PLC faults are easiest to diagnose from the front-panel LEDs and the diagnostic buffer. The LEDs tell you the fault family. The diagnostic buffer tells you the module, rack, slot, bus node, or program event that caused it.
Common Indicators
| Indicator | Meaning | First check |
|---|---|---|
| SF | System fault | Diagnostic buffer and affected module |
| BF | Bus fault | PROFIBUS or PROFINET node and cable |
| STOP | CPU is not running user program | Mode switch, program fault, memory |
| BATF | Battery fault | Backup battery and retentive memory |
| DC5V | Internal 5V supply issue | Power supply and rack load |
Step-by-Step Fix
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Read the diagnostic buffer. Use STEP 7 or TIA Portal before cycling power, because the newest event is the best clue.
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Match the slot or node. If the buffer names a rack, slot, or bus address, inspect that exact module or device first.
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Check power and grounding. A weak 24V supply or poor grounding can create intermittent SF and BF faults.
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Inspect bus wiring. PROFIBUS faults often come from termination, shield bonding, connector damage, or one offline node.
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Compare hardware configuration. A replaced module with the wrong order number can keep the CPU in fault.