Hello I have a C230 2 door 2003 and I am having the red screen with battery sign come up and now my key remotes won't set alarm and lock and unlock doors. Doors lock and unlock by pressing button on dash and I have put new batteries in key remotes and checked fusses 8, 9, and 41 for alarm and all good. Had battery checked and charging system and was told all good but that my voltage regulator was not good. Could this be my issue for alarm and remotes not working? I know I will need to replace alternator now, any ideas? Thanks in advance.

 

I would check your remote batteries again. Batteries should be 2025 with + side up. If the remotes are functioning the red check light on the remote should light up. If all is OK then either the car battery or the remote sensor(s) could be faulty.
Based on your description and remotes are double checked then car electrical system must be at fault. Car battery must have enough power to actuate locks and alarm.
Good luck