Started my '15 S this morning-no problem using CAccess. Stopped 10 miles away and it would not lock with handle. Tried fob-no success. Used key on columb to start. Only idiot message was that comfort access not working. Changed, checked battery-old one 3.05V. Tried other fob, same and would not work. TMPS works OK. Trunk will open from inside button only. No errors on Foxwell scanner. Any Suggestions? I can get a CA module for $350, but not returnable.

 

Had this same problem a few weeks ago. The head unit didn't even indicate that the battery was low and needs replacement.
Both FOB's didn't want to lock/unlock the doors. Changed the batteries on both FOB's. Also disconnect/reconnected the car battery. Even bought a new Radio Receiver to install.
But doesn't seem like I needed that receiver. I got 2 more batteries. Sat right next to the car and replaced the battery. Then one that FOB worked. Did the same to the other FOB and it worked.
That problem hasn't happened again so far afterwards.
Guessing sometimes these batteries for the FOB just are bad. But it was strange that the car didn't indicate at all that there was any issues with the FOB's.

Just for reference I got over 104k and it's a 2015 S.

 

Spotted,
Thank you! It worked. For $4, I'll keep a couple spares in the car. You saved a 45mile trip to the dealer.

 

Glad it worked!