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If the green key light is flashing, then there is a fault with the immobiliser system or key.
I have known a few to quite literally lose their coding and need the key reprogramming, this usually involved Honda diagnostic system and obtaining a pcm release code off maris (security code from Honda database using vin number).
I fear that there is not a lot you will be able to do without diagnosic equipment unfortunately as you would at least need to know the fault code and either a main dealer or a key programmer to do the job.