Might be splitting hairs but I think any decent shop would guarantee their labor on your part. However they are only guaranteeing just that - THEIR LABOR. So, if they put it in wrong they will make it right. If your part fails they are not going to redo their labor because you bought a defective part. Same thing if the part you chose to replace doesn't cure your problem. They are not guaranteeing your diagnosis of what needed to be done.