Hello;

When all LEDs start flashing together on a S7-400 CPU, it is a bad sign for the status of the CPu.

While the CPU is working (before the LEDs start flashing) open the diagnostic buffer and see what the controller is complaining about.

Here are some things you con check, if the diagnostic buffer cannot give you suggestions on the source of the problem (use the Save As button in the diagnostic buffer page to create a text file with the full contents on the buffer, this will allow you to save it and analyze the buffer even when the CPU stops working).

- change the memory card you are using for one of similar type and size;

- change the power supply on the rack;

- swap the rack for a similar one;

You will find more suggestions from the forum users if you use the Search box at the top of this page, looking for "S7-400 all LEDs flashing" in the Forum section.

If nothing helps, the internal memory of the CPU might be defective, and then you have to think of :either replacing the CPU or sending it to Siemens for repair (and hope it can be repaiored...).

For example:

 https://sieportal.siemens.com/de-ch/support/forum/posts/flashing-all-leds-on-the-cpu/27801?cl=en 

Hope this helps,

Daniel Chartier