Service Engine Soon

[Shaggylocks]

I have a 05 Standard Cooper with 60000 miles and the Service Engine Soon light popped on. It came on after filling up and I accidently put in 87 octane fuel. After checking MINI manual it said that there could be an exhaust issue or something as simple as the gas cap not on properly. If it ends up being an oxygen sensor what is the repair run usually? Or is it possibly another issue entirely?

[bad venge]

It MIGHT be a knock sensor out of range get the code scanned ... if ir disappears after 6/8 key cycles it might be that