Oops - we've out-smarted ourselves. Originally that LED was labeled "Status", to match the silkscreen label on the PMDX-126 board. Somewhere along the way (long before the PMDX-424 existed) we changed the text on the PMDX-179 panel to say "Error Code" but didn't update the PMDX-179 manual, which in table 5 on page 8 still calls this "status" (well, calls the corresponding signal on J3). The change to "Error Code" is because when used with a PMDX-126, that LED really only turns on when there is an error, or when the test button is pressed.
However, when the PMDX-179 is used with a PMDX-424, this LED really is (mostly) a status signal. A slow flash (on for 1 second, off for 1 second) means that the PMDX-424 is is running normally and communicating with the SmartBOB plug-in. One short flash every 2 seconds means the PMDX-424 is running but not communicating with the PMDX-SmartBOB Mach4 plug-in (usually this means that Mach4 is not running). Any other flash pattern indicates an error and you then need to look at the LEDs on the PMDX-424 to determine what caused the error.
Bob