I've heard people on this forum say that an exhaust leak at the front of the cat would cause problems with the O2 sensor readings, but I never understood how. With the engine running there should always be positive pressure on the exhaust, which should prevent air from the exhaust leak getting in (that would require negative pressure). I wouldn't think it could create a venturi effect either, because of the positive pressure. But what do I know? I can't get cheap Mevotech ball joints to work right.