Simple question - how do I change the battery on my 623 key fob ? Doesn't do the necessary so assume dead battery in the fob. Will battery change mess up the programming ? All help gratefully accepted.

Which one, the single button IR or dual button RF fob?

They both take 2 CR2016 batteries.

They both open the same way, use a flat blade screw driver to pry apart the fob from the keyring end slowly and it will come away in two halves.

The IR fob I wouldn't worry about loosing the code, the RF fob on the other hand can be a bit of a problem if it looses sync so don't press any buttons away from the car.

The book method of resyncing is to unlock the car with key and keep pressing lock on the fob and pray/hope it works.