Modus key battery low

Sounds like a classic low battery issue in the key fob. Those symptoms – keyless entry failing, error messages popping up, and inconsistent locking/unlocking – are pretty common when the battery voltage dips. Have you tried replacing the battery in the key? It's usually a CR2032. Also, does your Modus have a backup slot to insert the key? That could help in the meantime. Did you ever measure the voltage?

I haven't replaced it yet. I wanted to check here first. I didn't measure the voltage either. Where exactly can I find the battery type in the key? And where can I buy a replacement?

Okay, makes sense. The battery type should be printed on the battery itself once you open the key fob. Usually, it's a CR2032, as I mentioned, but double-check to be sure. As for where to buy one, any electronics store, supermarket, or even a gas station should carry them. Given all the issues you're experiencing, I would suggest visiting a workshop. Seems like the car needs to be reprogrammed after replacing the battery.

Thanks for the help! I got the battery replaced (it was indeed a CR2032). However, I had to visit a local workshop as suggested. They told me the key had to be reprogrammed after the battery replacement. Cost me 37€, but now it's all working perfectly again.