Do I just need the panel or will I have to get the key pad too
Also could I leave the back plate on the wall and change the wires like for like on the circuit board then put the g3 front panel back on so in theory I'm only changing the circuit board
Issues like this require you to get a multi meter and do proper fault finding, you can't just assume the panel is faulty. You'll end up spending money on new equipment only to find its not the panel.
BTW it's probably nothing to do with this "anti code" you've referenced.
Disconnect the bell connections before you apply the mains
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