2000 ford focus lx

okay my roommate has a focus that would not pass inspection last year took the car and had both o2 sensors replaced and it passed inspection no trouble since till about 2 or 3 months ago it started skipping replaced plug wires all was good for about 3 weeks started skipping again. today after getting gas check engine light comes on no gas cap i checked went to advance auto where i work and pulled code po136 for both of the bank 1 forward and rear o2 sensors . how what would cause year old sensors to go bad? we just put some injector clear in ti and its running better but still wondering about the sensors

Your post is very hard to read...

well its hard to tell . its very possible that a new O2 senser can fail... but sence both are going out at the same time it sounds like something more complicated,. you could replaced the O2 sensers and see if it was just a machincal failure and then you will know its not them