My XUV590i S4 works fine one day and shows the dreaded wrench the next, wait a week and its fine again. It appears to me that when the trouble light (wrench) is on that it is in limp mode as it has very low power. When the wrench is on it idles fine but if I let it idle long the muffler gets so hot it glows orange. I saw one post that was similar that said it might be the O2 sensor, but the intermittent part has me baffled. Please don't tell me to take it to the dealer, that is a huge hassle and expense. This JD has very few hours and miles and should not be having this problem.