My first post. From Northern NH and have owned several Jags and Land Rovers. Happy to join. Can’t wait for replies. I’ll try to be brief.
I purchased my 2007 Xk8 convertible on 11/18/23. Nice car with 70k miles. First drive, 10 miles from Jag dealership. I parked the car for 1 hour. Upon returning, car would not start. Repeated starting process several times as instructed by sales rep and manual. Was able to reach sales rep who asked me to repeat process and pay attention to dash screen. In first start position all lights and normal functions were indicated including check engine icon. Repeated on/off procedure. Did not start. Decided to try different combinations. Oddly, when I buckled the seat belts, car started. Being uncomfortable with driving 100 miles home, I called dealer. They picked it up with platform truck and it is now being diagnosed. The free CarFax indicated “checked electrical system” several times within the last 2 years. Battery was checked and changed 2X since new.
Service tech called the next morning to ask exactly what I noticed. Service tech called next morning asking me to explain my actions and observations. He did not know of seatbelt buckling solution. However upon attempting to start car, it started immediate without seatbelt buckled.
What may have happened. Could it be improper battery replacement, previous owner improperly jumpstarting. A faulty relay or systems glitch? A puzzle for sure. Opinions will be helpful as I discuss with service tech at dealership.
thanks in advance.

 

Seatbelt may just be coincidence. A possible scenario (it’s happened to me and others on this forum), is that the brake system got pressurized by stepping on the brake without starting the car. To get the car started, you now really need to stand on the brake pedal to get the switch engaged (really really stand on it hard).
The best way to normally start these cars, as noted in this forum many times, is to press the start button first, and while holding it step on the brake. Not mentioned in the manual, and most other cars won’t start with that sequence, but it works for these cars, and avoids the need to stand in the brake pedal.

 

It's easy to inadvertently step on the brake pedal ("pump the brakes") when sitting in the car with the engine off. Doing so raises the brake pedal and makes the car hard to start.

This problem was solved in 2011. Simply press and hold the START button before you step on the brake. For details, see:
https://www.jaguarforums.com/forum/x...se-time-57460/

Don't underestimate the possibility of a weak battery, especially now that the weather is getting colder. The X150 is a battery hog, and a weak battery causes all sorts of electrical glitches. Many Forum members use a battery maintainer to keep their battery fully charged, especially if it isn't a daily driver.

 

Terminology matters. Your 2007 convertible is not an XK8; it is an XK. The XK8 model years are from 1996 - 2006 and their factory model designation is X100. The X100 Forum is here.

X150 is the factory model designation for the all-aluminum XK/XKR model years from 2007 - 2015.