This is not a common problem as far as we are aware, but one of the keys operates normally and it is only the spare key that fails to hold the code then that would suggest the issue lies with the spare key and not the receiver. Replacing the battery is the obvious first step but given that has not solved the issue it may be that there is a drain that is causing the key to continually operate. We would suggest trying an automotive locksmith who will be able to examine the spare key and hopefully diagnose a fault, failing that we would suggest replacing the spare key - again an automotive locksmith will be able to do this more cheaply than a main dealer.