Hello John,
Did you need to reset the computer?
I found this information , hope it helps.
Kind Regards,
Julie.
How to Reset the Computer on a 2004 Jaguar XJ8
Step 1
Start with the ignition key in the "O" -- ignition off -- position. Press and hold the rear fog lamp switch.
Step 2
Press the "Start" button -- as though trying to start the car -- without putting your foot on the brake; this will put the car into "Power Mode 6." Immediately release the fog lamp switch.
Step 3
Press and hold the computer display cycle switch, then press and hold the rear fog lamp switch again. Continue holding the two of them while the system resets.
Step 4
Look at the display. If the process has been successful, the message center will display "Resetting Service Mode" for 10 seconds. After that time, it will read "service Mode Reset."
Step 5
Release both switches and remove the key from the ignition.