The battery in the key is used to power the signal used to lock/unlock the doors. It has nothing to do with the start sequence; that communication occurs via a transducer in the key.

Check the lights beneath the RR mirror and check to see if they are flashing (together, or alternating) using the bad key.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)