I hardly ever use my 2nd key, and on Sunday tried it for the first time in months, and it wouldn't unlock the car remotely, after opening the door with the key and switching on ignition it started working.
Does anyone know if the keys 're-sync' somehow when ignition is on and remote lock/unlock is pressed ?
I'm not sure if the battery was flat (but its working fine now, and I can lock/unlock the car from about 100ft away as normal), or maybe just dirty switch contacts in the key so didn't work at first ?
Anyway, searched on here to find the correct battery for this and couldn't find it, so for reference its Panasonic CR1616 3v cell or equivalent - if you search ebay for CR1616 there are tonnes on there starting at about 10p each :-) :-) :-)
IIRC the keys are of the rolling code variety and the car will only accept the next expected 3 or 4 codes from each registered key. So, if the button was pressed a few times (like playing with your keys) then the car doesn't accept the next code as it wasn't expecting it.
It also appears that if the car "sees" a correct sequence of codes (and I have no idea how many) then it starts working again. <shrugs>