Put it on a techII and rear o2 sensor was bad, replaced it. 2 days later p0171 popped up. Front o2 sensor was bad, replaced it. 5 minutes later p0171 is back. Sensors are reading normal. Bad Cat? Intake gasket? I swear I can hear a vacuum leak but smoker reveals nothing. Anyone else have this problem? Car has 117,000 miles and this is the first issue it has ever had, other than factory replaced cat at 75,000 miles. Any help is appreciated.