Most car radio or stereo comes with a code from the manufacturer of the car in order to deter theft, and when the car stereo is detached or removed from the vehicle, or sometimes when the battery of the vehicle is disconnected, the radio cannot be used unless the unlock or security code for it is entered.
And even if the serial and model number of the stereo is obtained, for the code to be unlocked as well from the manufacturer, this will still need the VIN of the vehicle for the code to be provided and be used so that the radio will be functional again.
In obtaining the code, you will have more than one option, and how it is obtained also depends on complexity and cost, that if you are in a situation where such predicament have to be addressed.
You may have the option to check the user’s manual, check the OEM’s website, call your local dealer, search through free online databases and they will need info. like the make, model, year, and VIN of your vehicle together with the serial and model of the radio, but this is somehow a courtesy service that they are not obliged to provide.
If none of the above works you can always opt to pay a local shop or online service to locate or calculate the code, and these days this is the most popular and efficient access, where they are able to generate and provide radio codes of almost all car manufacturers using the serial number of your radio and such service though requires you to pay a certain amount in order to get the code and have your radio functional again.
Nonetheless, if you are the car owner you will have to carefully input the code and know what is the procedure or the functions on the type of radio that you have, especially if this is your first time of inputting a code, chances of inputting the code by mistake multiple times may lock you out and will require you more time to reset the radio and wait before you will receive the prompt to input the code again.
Just accessing the code may be a daunting task, therefore if you happen to have that code, keep it safe or noted safely where you can have direct access to it personally, so that you will not have to undergo the same trouble again should it recur, and you will not be paying again for the code in case it was bought online, however there are some online services that offer free radio code recovery for your code that was previously ordered from them, so you need not pay again for the same code.