HI

I have got a dellemma, I have replaced the post cat. O2 sensor 3x now-the third time with an OE sensor. After the engine warms the CEL comes on, the voltage is good at start-up. as are the wires/connectors, the flexpipe was just replaced. The valve cove leaks oil severly and I was told that could set off the light, due to the fact that the engine computor always read a rich mixture need from the front sensor. Is it possible that the PVC system/valve which is in the valve cover(another on order) could be sucking in air to register the need for a rich mixture to the front sensor and not burning the risidual oil in the valve cover as designed to do and inturn sending this faulty message(need for a rich mixture) to the rear sensor since the front sensor is not reading the mixture need correctly? When I replaced the rear sensors they came out very carboned up. After about 30 mins. of driving after the new sensor is in the CEL comes on-which could possibly mean that the sensor never kicks the voltage down to a leaner mixture-any help much appreciated. Car is a 2001 hyundai Elantra
Is this a possible senario?