AWS offers transient instances as a way to sell unused capacity in their data centers. Although these transient instances come with a steep discount at their prices, up to 90% of discount compared to on-demand instances, they do not come with availability guarantees, thus, they are not reliable for regular usage on the Internet. This paper presents PISTIS, a framework that uses reliability engineering models that provides reliable computing resources over transient instances by proactively replacing components that are expected to fail. We evaluated our model and the results show that we can achieve the same reliability of regular on-demand instances but with a price reduction of up to 81% in the best cases compared to on-demand instances.