fiat punto radio code

Hi I have just bought a replacement radio for my punto mk2 complete with serial no and code when I fit it all I get is a message saying check I have looked
in the manual and it says that the car computer will check the radio then I can enter the code, but it just says check and never changes to enter code am I doing something wrong or is there something else I have to do, can anyone help me please
many thanks colin
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