O2 sensor

I have a 94 530i with 100,000 on a new block. Recently the check engine light came on, and through the blink procedure found out that hte fault code was 1213 which is labeled at O2 sensor #2 I replaced this sensor and after a while of driving the engine light went off. Now 2 weeks later the check engine light came on. Read the code and now it`s saying O2 sensor #1. Some how i feel that it is not the oxygen sensor which is the problem. Any suggestions other than checking vaccum lines and the usual procedures.
Thanks