Heated rear screen button on when starting.
 

Just though i throw this question out there. Lately when i start my car the rear heat screen button comes on. I press it off and it stays off. It keeps doing this when i start the car any time. Any answers and before anyone says anything , it's not a battery issue. it's 100%.

Same thing happened on mine for both front and back. I believe it happens when the car senses a low outside temperature.

Didn't happen when the temperature was slightly higher than 0.

Had thought about that, maybe this is why, snow and frost here this last few weeks, bitterly cold, good call. Will keep an eye and see will it stay of when the temp goes up.

Yep same happens to me, its just built in, its useful though sometimes it does get it wrong when the temperature is low but there is nothing to clear. So its just a guess made on the temperature

Thanks guys, first winter with this car, my old 04 xk8 hadn't got this fuction, just press on and off, but it had a heated front screen, don't know why it isn't on a 07 model, just a defrost button.

It´s working from 5 degrees celsius or lower outside temperature.

Yes, this is an automatic feature; the mirrors also are heated automatically, and the front windscreen if your car has this feature.
Now, if you use your XK in winter conditions and for relatively short runs, you can understand why the battery discharges! If you have a garage with electrical source, a CTEK battery maintainer is the cure for any electrical issues caused by low battery voltage (and the symptoms are varied and random).

That sounds right with the 5 deg C, mine seems to come on automatically around when it's 40 deg F out.

I find it annoying more than anything as my rear window has yet to be fogged up. Even this afternoon I left a customers plant and it was 11 deg. out. Rear window was clear even though outside for 8+ hours.

Also wonder if that's part of the cause for the rear window separating on verts. Seen a few posts in the forum about it. I know there isn't a timer on the defroster as mine doesn't shut off unless I do it manually. All I do know is every time I start the car the first thing I do is shut off the defroster as it's winter here.

From the manual:

I've checked, and it applies to the 4.2 as well as the 5.0

Good info Ngarara,

Must have missed that in the manual, thought I read it cover to cover.

Guess I haven't been in the car for more than 21 minutes or at least turned it off long before that.