I can't speak to the XK, but have extensive experience with the XF:
Start your test-drive looking for warning lights and messages in the XK’s instrument cluster, which could be the result of a poorly-charged battery. Low battery voltage can cause a world of issues in a car like the XK, many of which aren’t typically daily driven.
Yeah, Lady Noto's dad's XF would fail to start if not driven within 4 days. This happened ALL THE BLOODY TIME. The battery is located in the trunk as well, as an FYI, so if you back into your garage like a good lil' boy or girl, it'll be much more difficult to boost.
![](http://news.advancedbatterysupplies.co.uk/wp-content/uploads/2013/10/xfbatterylocation.jpg)
Though rare, some owners have reported failed catalytic converters, possibly evidenced by a ticking or tapping sound as the car travels over bumps.
We experienced this as well. Replaced under warranty and no recurrence of the problem.
Overall, we loved the 'Jag' and didn't find it to be unreliable despite the above. The 5.0L V8 is great, but its fuel economy and the massive overall size of the car compared to tiny living quarters made the car too difficult to live with.
...hence why I'm excited for the XE - a smaller Jaguar!!!