The Unhappy Kitty - Jaguar Check Engine Light On This is a great study of dedication, and a solid illustration that logical thinking, some "detective work", and understanding of how automotive systems actually work, make all the difference in service outcomes. In other words, "knowing", rather than just "parts changing". It's also a cautionary tale that "scanning the computer" and use of fault codes, does not fix cars, it may or may not give an indication of the area in which to investigate, but unless there is true knowledge of how the system operates, it can lead to unnecessary work and parts replacement. [...]
A very unfortunate but very common Jaguar fault, a broken secondary timing chain. With less than 78,000 miles on the car, in this case it appears that the plastic timing chain tensioner guide had come loose. The loose tensioner guide was then caught between the chain and the sprocket, causing the chain failure.