Hi, I m having the same issue for downgrading from 4.5 to 4.3.

The CPU is a 1214C 6ES7 214-1AG40-0XB0  FS 13  

I can upgrade but no way to downgrade. I have checked the the link and i see no notice or whatever that could explain the rootcause. 

https://support.industry.siemens.com/cs/de/en/view/107539750

I have a project developped by a 3rd party that was done with TIA Portal V15 and 1214C with defined fw 4.3. My custommer has told me i have to work with this version and now way to leverage to fw 4.5.

All the Hw 1214C i have received has fw version 4.5

Note: I have tested an upgrade (4.6) and i have no issues.

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Split from Cannot downgrade S7 1200 1212C CPU From Firmware V4.5.1 to V4.2.

Hello Sugreev,

CPUs with FS 13 come with firmware V4.5 installed at the factory and a firmware lower than its factory firmware cannot be installed on a CPU.