Hi @RFC7, The key fobs can be a pain even to the service people... They need a computer to match it with the central body controller. Sometimes it's a 10 minute job other times it can be nearly an hour... It's all in the software... Good luck, Shawn
I had the same thing happen to my key fob. I am about 150 miles from the nearest service center, and had literally just had my annual service done a few days prior when it happened, so I was really trying to find a solution without going all the way back to service. Finally stumbled upon a site in Ukraine that shows there is a weak point between the battery connector and the board that sometimes breaks when putting the new battery in. I hit this with a small dab of solder to bridge the battery connector and the board test point/trace, and the key started working again. Hopefully this helps someone save some time/money in the future if they have the equipment/ability to fix this themselves.
I had a similar problem just recently with my Model S. The batteries in both my fobs were getting low, so I went to replace one (even used the recommended Panasonics) and post-replacement, the fob wouldn't work no matter how many batteries I put through it. I put in the Energizer from the other low-battery fob and it started working again. Looking closely, the ground pad on the Energizer sticks out further than the Panasonic (and Duracels that I also got thinking I had a bad batch of batteries). On the outside edge of the ground spring pad that lays down underneath the battery, there was a bit of metal sticking up where you'd normally pry the old battery out. This was apparently shorting out any shallower battery I put in there and killing them within an hour or so. A little adjustment with a screwdriver and both fobs work good as new with any of the batteries I had on hand. I suspect it's relatively easy to pry up that little piece when removing the old batteries, so that's probably what did it and is why removing the old battery "kills" some of these fobs.
I having this same issue - low key fob warning and doens't work at all after battery replacement. Worse yet I had the dumb idea to try swapping batteries with the other fob and now neither fob works! To top it off the app is acting wierd and says unlocked but the car is clearly unlocked. I can enable keyless driving and roll down the windows to somehow get in but this doesn't seem like a great way. I've put in for mobile service - luckily they can be here in 2 days on monday - would mobile be able to fix all these (Keyfob v1.0 Late 2016 Modes S). I'm going to check the contacts as suggested by the post above. If I didn't have another car I'd be stuck completely at home!
update: I tried removing and repositioning the battery while standnig outside the car with both fobs, someone mentioned the contacts to check they hadn't lifted up causing shorts etc. Anyway both fobs are working! Not sure exactly what I did
I don't think there is even an annual service for any Tesla any more. At least that is what they told me about my car when I went in for the HW3 upgrade a few months ago. The car is almost 4 years old and never been in for annual service. They keep telling me it isn't needed yet.