P0135 Code keeps returning

So I replaced my upstream O2 sensors about 2 years ago and about a month or two ago I got the P0135 code (bank 1, sensor 1). I replaced the bad O2 sensor and the code went away. Fast forward to yesterday and the P0135 code came back on... WTF? Is it really possible that my O2 sensor went bad already??? Is there a way to test them to see if they are working so I can decide if it is an issue w/ the car or an issue with the O2 sensor?

Thanks erebody

Update:

I looked under my car and noticed that the O2 sensor is slightly bent from the base. It looks like it might have touched the K-member and bent slightly becuase there is very little clearance between the sensor and the k-member. GD Magnaflow placed the O2 sensor in a horrible location...

Do you think I could just cut out a quarter size piece of the k-member to give the O2 sensor some space, or perhaps just hammer a dent in it with my 2 lb hammer?

Does anyone know if they sell O2 sensors with a shorter tower on them? I don know the Bosch ones are actually longer than the Ford replacement ones

bump