on the key fob, turn the key over so that the buttons are on the back. there is a small groove near the blade/key end of the fob. you can prise this open using a small screwdriver or if you're strong pull the blade/key part outward/away from the fob. the remaining key fob can then be split in two to get at the battery. beware that you only have a very short time to replace the battery or the fob will go out of sync with the car.
Two wire connection in the fob. Open and check both wires are fix in place.
take the thug approach and rip the door off
you get 3 mins to change the battery.
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found this answer on this site and it worked for me "if you have replaced battery in remote key fob put key in ignition then turn to position 1 the press the buttons on the fob a few times. this simply resets the fob to the car then all will work as should do "
433.925mhz
Change the battery in your key fob. The directions should be in your owner's manual.
slide the top half away from the actual key as it has little locator lugs fitted good luck
Put key in ignition. Turn key till dash warning ligth come on. Press "open" button on fob. If it works the locks will go down and up to show it has worked.
Even if the trunk release is broken, the external key lock should be able to open the trunk.
The battery is a CR2032, Vauxhall part number 90541381.
You turn the key to the ll position or second click and press the remote pad to open the car. The central locking and indicators for the immobilizer should activate and that is the key programmed. Hope this helps.