My 03 Xtype 3.0 with 86000KM went up red light " Security Failed" and no cranks after I use the key to open the door this morning.
But everything works fine.
I tried to remove the positive side and reconnect the battery after 3 hours, it still showed Security Failed.
Is it the ECU problem?

 

When you turn your key to full on (not start), does the MIL (little engine icon) remain lit or does it extinguish?

 

the Check Engine light was gone.

 

If the MIL extinguishes (goes out when key is in full roll not started), that is a Jaguar signal that the immobilizer is activated. Do a search for that problem here in this forum for guidance.