Why is the red I/O LED ON on a M241 controller?

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Why is the meaning of the red I/O LED ON on a M241 controller?

The red I/O LED indicates device errors on the serial line 1 or 2, SD card, cartridge, TM4 bus, TM3 bus.

For example, only the fact that serial line 1 or 2 is not connected or the SD card is not present will turn the I/O LED ON but everything else may work correctly.

To find out exactly why the LED is turned on, you can check the PLC_R structure in a Watch window, specifically PLC_R.i_lwSystemFault_1

Bit field FFFF FFFF FFFF FFFF hex indicates no detected error.
A bit at low level means that an error has been detected:
- bit 0 = Reserved
- bit 1 = TM3 detected error
- bit 2 = Ethernet IF1 detected error
- bit 3 = Ethernet IF2 detected error
- bit 4 = Serial 1 in overcurrent detected error
- bit 5 = Reserved
- bit 6 = CAN 1 detected error
- bit 7 = Reserved
- bit 8 = Reserved
- bit 9 = TM4 detected error
- bit 10 = SD Card detected error
- bit 11 = Firewall detected error
- bit 12 = DHCP server error detected
- bit 13 = OPC UA server error detected