Changed the battery in 2003 Volvo S40 now radio says off and I have no codes

There is a security code that you received with the vehicle that has to be input to the radio to reactivate it. If you don't have the code, you will need the dealer to reprogram it for you.

You would need to call Volvo parts department to see if they will give you the code. Mdost likely you will need to go down so that they can get the serial number to get it for you. Once you get the code, when you get home you need to leave the key on with the engine off. It could take several hours or maybe less but this will reset the radio to allow you to input the radio code. If you have a battery charger you may want to hook one on on very low charge while the key is on so that you do not drain down the battery while you wait for the radio to reset.