OBD says 02 sensor left bank,had it done at autoparts store, didn't know if front or back one...engine runs /idles great..just passed CA smog 30 days ago.

is this a DIY and is it the back 02 sensor..

 

What is the specific error code?

Often there is a series of tests to perform to eliminate some other cause other than the sensor. However, the other causes -- associated with wiring, or even the DME -- are often less common than just a bad O2 sensor and if one doesn't mind the small risk of throwing in a new sensor when the old one is just fine you can replace the sensor just based on the error code.

However, if the sensors have been service for the same amount of time I would recommend you replace the sensor on the other bank, not for fear the other sensor will also go bad but to avoid having one bank controlled by a new sensor and the other bank controlled by an older sensor.

Oh, the sensor job is a DIY. There are some things to remember: Handle the sensors with care, especially the new sensor(s). Do not touch the sensor tip! Do not attempt to measure the resistance of the tip. Do not use any thread lube on the sensor's threads. Do not use any electrical contact cleaner on the tip or on the sensor connector or the wiring harness connector.