To anybody that can help,

My alternator recently failed so I had it replaced. It was supposedly working fine but the first time I took it for a drive it stalled every time I stopped or the rpm's got too low. I do have someone that can fix it for me but I cannot afford to buy random parts until I've found the problem. I took it to orielly's and got a reader code of p0421. Please help. I am a mother of 2 and a college student so this is a desperate situation. I appreciate any and all feed back. Thanks!