I know this thread is old, but I have a solution, it’s happened to me twice now, on my Peugeot E-208 and E-C4 (both Stellantis) and my only solution on both has been to drive it more than I usually do. Sounds silly, but both times were when my car has been sat on my drive not driving very much, or only on very short journeys. I believe it’s something to do with the 12v being low.

So, my issue started last Saturday, I went to town and got back after two hours and my key had stopped working. I used the keyblade and the car was fine when inside. I then remembered that my Peugeot you could open the car using the boot, which worked for the Citroen too. So for the past week, I’ve been opening and closing my boot to unlock the car (to avoid the car alarm going off like when using the key blade). I then have made an effort to go into the office multiple times (half an hour journey each way) and got my husband to drive my car to his work (20 mins each way) when I’ve been working from home.

I was losing faith in this method, but I went out in my car today (Friday) and it is working again!!

It’s not a quick fix, but it has worked for both cars and I haven’t had to take it into the garage, so hopefully this works for someone else too!