I think eventually these things just go bad but the design to change this heater core out is awful! I had to take the entire dash out of the truck just to access the heater core box. It is a real pain in the butt. I said I would never do it again and was worried it would not go back together correctly. The part available at Autozone is not an exact match but fit with little modifications. It is not fun and not easy. If you are not skilled in working on this type of problem, I would not suggest working on it yourself. If you are good at this stuff it will take you a while but can be done with time and attention to details.