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Beverly Diaz MDJul 26, 2025
What to do if none of the CPU LEDs turn on my Siemens SIMATIC S7?
- DDiana WilliamsJul 26, 2025
If none of the CPU LEDs on your Siemens Controller turn on, a possible cause is a blown fuse. Use a line analyzer and monitor the input power to check the magnitude and duration of the over-voltage spikes. Based on this information, add the proper type surge arrestor device to your power wiring. Another cause may be reversed 24 V power wires. Refer to the wiring guidelines in Chapter 3 for information about installing the field wiring. It also may be caused by incorrect voltage.
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