2015 A3 spare key fob doesn't lock/unlock car

[Rcord]

After keeping the spare key fob without use for years, I decided to test it to check if it would work in case I needed, but it didn't. It can't lock/unlock the car. Battery doesn't seem to be the issue because the LEDs in the key fob blink normally when buttons are pressed and, after all, the battery in the other key fob I use every day is still working perfectly. I tried the resync procedure from owner's manual (press lock, unlock driver's door using the key, press lock or unlock) but no success. Also tried pressing unlock while turning on the key in ignition but didn't work either. Any suggestions?

 

[malteso]

Start with changing the battery. Led might light, but may not be enough to transmit.

Sent from my LM-V405 using Tapatalk

 

[NHN]

Swap batteries between fobs to test, easiest option as one does work, if old fob battery works in daily used key, then probably needs diagnostics to setup unused key again.

 

[Rcord]

Thanks all for the replies. Now I noticed that car settings on MMI displays "Number of keys initialised: 1", and I remember I used to see 2 on that screen a few times I checked since I have the car. It seems that, for whatever reason, the spare key lost its adaptation. I've read that it can only be coded by an Authorised Service with an online ODIS connection, but if someone knows how to do it with VCDS or VCP any help will be greatly appreciated.