I had the battery replaced on my xj8. Now the radio won't turn on.

 

It sounds like your radio has a security code, Most US cars do not have a code, it is usually a European thing, but I've heard that some US cars do have a code.

I think if you call a Jag dealership, and give them your VIN they can give you the correct code.

 

+1 Some of the cars running around the northern states were originally sold in Canada: most of those required the radio code. But look in the glove box, the code was often found with the drivers' manual and related documents. My first X308 was a Canadian: code was with the warranty . . .

 

I disconnected the battery cables and put them back and the radio works.

 

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Electrickery!

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as stated in this thread, if you lose power or disconnect your battery the radio will not work without a four digit code, which usually comes with the radio handbook or with the warranty information.