Peugeot 407 Key Battery Low Warning

This sounds very familiar. I had similar issues with my older Volvo V70; same 'low battery' warning, and eventually the key fob stopped working altogether. Think of it like a small flashlight; the bulb dims before it dies completely. Have you tried replacing the battery in the key fob? It's usually a CR2032 button cell.

I haven't replaced it yet. I thought maybe something else was wrong since there's an error stored. Did you have an error too?

Yes, the error message was related to 'key fob communication failure' or something along those lines. Replacing the battery resolved it. The error might need to be cleared with a diagnostic tool, but replacing the battery is the first step. If the battery replacement doesn't fix the issue you may want to visit a local workshop to have a diagnostic run, but in my opinion a battery replacement is the easiest and cheapest first step to solve the issue.

Thanks! Changed the battery. That fixed it. Ended up costing 37€ for the new battery and to clear the error code.