Siemens S7 300 CPU Status and LED Errors Explanation

In Siemens S7 300 CPUs, there is a series of LED lights with different colors which shows us the modes, status, and errors of the PLC.

S7 300 LED Errors

Siemens S7 300 CPU Status and LED Errors Explanation


System fault: it indicates there is an error in the system, this error could be software error like an error in programming or hardware error like a power loss of one of the input modules

Color: Steady Red


Bus Fault: it indicates that there is an error in the system network, like a bad contact in one of the communication connectors, or there is an overlap between the addresses in the network

Color: Steady Red


Maintenance: where ever this indicator energized this means that the CPU is not working anymore and needs a service, but this is rarely happening.

Color: Steady yellow


This led indicates that there is a 24 Volt DC provided to the CPU.

Color: Steady Green


Force: it indicates that at least one of the PLC inputs or outputs is forced to on or off.

Color: Steady yellow


Run mode is on whenever the CPU is working properly without any troubles, and flashes in the startup of the CPU.

Color: Steady Green



When the CPU is in STOP mode or there is a software/hardware error.

Color: Steady yellow


For a memory reset request when inserting the memory card.

Color: Yellow flashes slowly


When the memory reset is being carried out.

Color: Yellow flashes quickly

Author: Karim Ali Anwar

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