I lost key to car and bought new one from Jag car won't start. on 1991 Jaguar Vanden Plas

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Key goes in fine but car won't turn over. How do you get it to reset for the new key ?
(1) Answer
Do you mean it won't start(run) , or won't crank(where the engine actually spins). Does the car have any aftermarket security installed. When you bought the key from the dealer did they say , or did you inquire about , whether or not the key NEEDS to be programmed ?