You have 2 choices here: take it to a Subaru dealership and get them to diagnose and confirm it needs a new keypad module (which is what it sounds like), if the vehicle car battery goes flat the battery in the module will eventually drain and it will start acting erroneously. Or get the Brant alarm system removed without even trying to get it repaired. That's about it really.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i