Air fuel sensor

[covo71]

To the more knowledgable people out there. VCS, engine light etc came on. Got it on a scanner and it said bank1 sensor1 fail. 

I changed it over for a new one. All good car driving beautifully again for about 20 mins lights go on again. 

Wait a couple of days take it out again to make sure i clipped in wire connector properly and took the sensor out and it was black like the first one after a couple of days. I cleaned it up put it back in same thing 20 mins on the hoighway all good then the lights come back on. 

Anyone had to overcome the same thing? 

Ideas greatly appreciated, 

covo