Agree with Sebba on O2 sensor.
It should also be checked there's no air leaks into exhaust system down theway before the sensor, like through cracks, holes,damagedgaskets.
So if your flex is damaged and the sensor sits after the flex - there your problem is. Fix/replace the flex and voila.
Still at this mileage you sensor could be pretty much asking replacement.

 

for a deeper claifacation of the oxgen sensor. It's senses the oxgen content in the exhaust and adjust the fuel mixture ratio from the fuel injectors searching for the most efficient burn, less emissions.

Sounds like the diag is correct about the flex pipe leaking, if ox was getting in, and the sensor is downstream, could cause the sensor to adjust in the wrong direction. Plus, if you have an exhaust leak, you need to fix that, it can kill you if it gets into the cockpit. (notice any headaches lately?)