P0455 and P0456 on you Jaguar X-type

The code P0455 and P0456 is flagged when the computer detects a leak in the EVAP system. What's the EVAP system? In your fuel tank while you drive the car and move the gasoline around gases form. Now by law this gases cannot be simply released into the atmosphere, so the car uses the EVAP system to store this gases and eventually (when the moment is right) feed them to the engine. The computer monitors that this system doesn't leak, and this code is flagged when the computer sees a problem. The computer has a little pump that creates a vacuum in the system, then with a pressure sensor sees if the vacuum stays. If the vacuum doesn't stay the computer flags this code.

MOST LIKELY CAUSE:

First of, if you haven't already change the fuel cap. Here is the link to the part: Stant 10851 Fuel Cap

Then just clear the code and see if it comes back. I changed countless fuel caps over the years.

If that doesn't work, you might want to look at the canister close valve and the canister purge valve. This are very common as well.

At this point if the code persists there is a leak somewhere in the circuit. This is when you might want to call it in. To test this, what I do is pump smoke into the system, and check where the smoke escapes to see where the leak is coming from.