I've had this problem since the first key fob battery needed changing 3 years ago. Since then I've had to renew the battery about 10 times. It is not the key fob battery at fault, it's something in the car. My wife also has a key fob to my car and she has the same problem with it. I've changed her battery half a dozen times. Sometimes it operates the door locks perfectly, sometimes it just doesn't work. Sometimes the doors open OK but then I can't start the car because the presence of the key fob had not been recognised - "SMART KEY NOT FOUND". If this happens, I've learned by experience that if I press the key fob to LOCK the doors, they will lock, then I can start the car and then UNLOCK the doors. Other times, I get the "Smart key battery low" warning come on whilst I'm driving, the if I stop the engine, start up again, the chances are that the warning has cleared. This is driving me mad - any clues please?

 

1. Try asking in the XF Forum. This is the XK/XKR Forum.
2. Do you always take the fob out of your pocket? If not, the signal may be blocked by your house keys, cellphone, etc.
4. Is your car's battery fully charged? If not, a low battery can cause a variety of electrical gremlins.
5. If you buy your fob batteries for a low price on the internet, they may not be genuine. Asian-made knockoffs are common and don't last.
6. Do you leave your key fob in the car? If so, that will deplete the fob battery as well as the car battery.
7. If the previous owner was a government official, problem solved! Sometimes it works, and ... well, you know the rest!

 

Sorry for placing the query on the wrong forum. I'll put that right now.

Just to address your helpful suggestions:

1. OK
2. I keep the fob in my pocket whilst I'm driving, but otherwise I operate the fob in my hand, for example when locking and unlocking
4. The car (gel) battery is less than a year old. It holds charge enough to allow the eco stop frequently so I guess it's OK
5. They're Duracell High power 2032 coin batteries, fitted only 1 week ago.
6. Never
7. I was a government official for 15 years - but now I'm fully recovered!

 

No problem with posting in the wrong forum - we humans make misteaks.

I, too, was employed by the government but I left after 7 years. You had more patience than I did.

The Intelligent Stop-Start system in my XJ works off the small, secondary battery. I'd still get the primary battery tested, even though it's less than a year old. If you mostly make short trips of around 10 miles daily, your primary battery might not be fully charged, unless you regularly use a battery maintainer. I use a CTEK and have never had any battery problems.

Hope you find the cause of the problem.