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Asked:18 Oct 2025

SIMATIC S7-300 / S7-300F: Sm 336 Red Led lit up

Dear Specialists,
I have an issue with one of our clients with Failsafe AI modules.
The part number is 6ES7336-4GE00-0AB0.
The setup is redundant modules split by 2 ET200M racks.
The signal status connected to the modules are healthy and well received in the system, however the Red LED status on the modules gives some concern to the customer.
For your information, the diagnostic interrupt and wire break diagnostic has been activated since installation.
For spare channels, 4.7k Ohm resistor has been installed to prevent wire break diagnostic.
Current status from Simatic diagnostic buffer, there is “No fault”, no “Wire break”, “Module is available and healthy”.
Has you ever encounter this module behavior?
Can you tell me the reason for the Red LED lit up and the impact to the signal/system and how to make it disappear?
Thank you.
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Split from
How Safety Ai Module Led Off
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#sosflex #SIMATIC S7-300 / S7-300F #siemens
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Answers(1)
Siemens Test Added an Answer on18 Oct 2025

Failsafe AI Module (6ES7336-4GE00-0AB0) Red LED Status

Dear Customer,

The red LED on the 6ES7336-4GE00-0AB0 Failsafe AI module in a redundant ET200M setup can illuminate even with "No fault" reported in the diagnostic buffer. This is a known behavior and often indicates the module is in a 'safe state' due to the activated diagnostics, even if no actual fault exists.

Possible Reasons & Impact:

  • Diagnostic Activation: With diagnostic interrupt and wire-break diagnostics enabled, the module continuously monitors for issues. The red LED signifies ongoing self-test routines and safety monitoring.
  • Spare Channels: Although 4.7kΩ resistors are used, the module still performs diagnostic checks on these channels, potentially triggering the LED.
  • No System Impact: The red LED does not necessarily indicate a functional issue with the signal processing. The module remains available and healthy according to the diagnostics.

How to Address the LED:

  1. Acceptable Status: The red LED can be considered normal operation within a failsafe system configured with active diagnostics. Inform your customer that this behavior is expected.
  2. Diagnostic Configuration Review: If the constant LED is unacceptable, consider if the level of diagnostic coverage is truly needed. Reducing the diagnostic scope *may* extinguish the LED, but it will reduce the safety level. Caution: Modifying diagnostics impacts safety functionality.

Further Support:

For detailed information on failsafe modules and diagnostics, please refer to the following resources in SiePortal:

Sincerely,
Siemens Self Support

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