1043 code

I am getting a engine management system failure message on my dash and I check the codes. It is a left side 02 sensor. Checked the gas cap and cleared the code, drove it no problems. Drove it home today, and code came back.

So..... how hard is the rear o2 sensor to change? Have done them on other cars. Also with 24k miles it seems odd to me that the sensor would fail.

Is there something else I am missing?