Well I drove the car around after being down for a while, everything was fine. Shut the car off and went to drive it after a few hours and it was running very rich. Almost to the point that it would start to run rough at around 1200-4k rpms and black smoke coming out of the tail pipe. The current 02 sensor has around 15k miles and probably 25 galons of leaded race gas through it also. I checked it with a voltmeter and get normal reading at idle but at around 2k rpms it reads zero. My question is has anyone had an 02 sensor fail all of a sudden? I had thaught it might give me a warning before it went completely out.