I bought an used S40 and the seller only gave me one key to it. The radio says there are 3 keys, and of course the seller knows my address. So I've asked him if he has the other 2 keys to give to me, but I suspect the answer will be no, and I am worried (paranoid) that there is the chance he could actually have another key, and could therefore come by and easily open up my car at any time.

Is there a way to reset the lock codes on the key or something, to disable the other 2 keys?

 

My experience has been that only the dealer can do anything with the keys. I suspect you will need to purchase a new key and have that programmed to your car by the dealer in order to disable all the old keys.

And be careful with those keys you find on ebay. I've been told the dealer cannot reprogram them to your car without certain information that those ebay keys do not have.