If the Power LED flashes, it indicates one of two things:
- The controller/PLC is powering up.
- One or more of the I/O modules did not pass their self-check.
On most modules, a red LED will light up if the self-check fails. The LED can be seen by looking inside the controller/PLC from either the top or bottom side through the ventilation holes. For the PSM module, there is no internal LED. In this case, the Power LED itself will flash if the module is not OK.
The few modules that do not have this LED are: IOM3.1, IOM3.4, and PCM3.1. If the PCM3.1 is down, you cannot connect to the system and nothing works, so this one is easy to identify. For the two other modules, unplug one at a time and check if the Power LED changes from flashing red to steady green. In general, if a module isn’t OK, unplugging it will make the Power LED change from flashing red to steady green.
If needing to remove several modules before the Power LED returns to steady green, reinsert the modules one by one to identify which module(s) that causes the Power LED to start flashing red again.