I've replaced mine 5-6 times over the last 1 1/2 years or so. same one-driver's side every time. Ford never found the cause....they just keep changing them. Luckily, I think I've only paid twice...the rest were under warranty. One time they told me there was coolant sitting in one of my spark plug holes, and that was probably what caued it. They checked the entire coolant system, and found no leak....I don't get it. 12k since last pack change and all is well....keeping my fingers crossed.