The wideband sensor can't tell the difference between an exhaust leak and a lean condition. It just sniffs the amount of oxygen in the exhaust and reports back. It doesn't know where the oxygen came from since it is blind. You can't tell the difference between rich or lean with your nose since they both smell the same. A rich condition will burn your eyes and so will a lean condition. Best bet is to use the idle mixture screws to give you an indication of what the engine wants. Watch the vacuum gauge and the tach at the same time. You can also watch the wideband but don't necessarily believe the numbers, maybe just watch which way it moves.