Petrol. Looking on some us sites it seems it is most likely a dodgy sensor. It still runs fine, and I now have it booked in for tomorrow morning.
Not stressed about it yet, these things happen, but I'll keep an eye on how it's dealt with by the service dept after seeing of other peoples horror stories.
I found the tranny module had failed the day I picked mine up wombat, the local dealer was great and it was sorted quickly, it also had 1000 K's on it.
Like I said, these things happen, and I'm not trying to get stuck into them yet. But I am keeping my eyes open about the situation. So far so good, they managed to fit me in tomorrow, and I realize Saturday is probably their busiest day. And it's pretty short notice.
Would an O2 sensor be something they would stock? Or would it be a case of order it and wait?
Expect them to order it but as you say it is still running so waiting shouldn't be an issue, I had to wait two weeks for the module but it also wasn't a huge issue.
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What have you done to your JK this week?
Google tells me it isn't too much to worry about (error code p2099) probably a bad O2 sensor. 
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