GM-GMC: could it be a O2 sensor, gmc sonoma, catalyst system

Question
I own a 1996 GMC Sonoma SLS 4cylinder 2.2 liter 2 wheel drive.  My engine light came on so I took it to a local parts store, they hooked up a code reader and it returned a code of P0420 catalyst system efficency below threshold (bank1). They told me it may be my pcv valve, so I changed it out, within a day the light came on with the same code.  I am wondering if it could be an O2 sensor before or after the catalytic converter, any help would be greatly appreciated. Thanks in advance. Clayton

Answer
The code comes back as Catalyst System Low Efficiency, but with no bank reference.

The O2 sensor(s) cost less than the converter, so I would try replacing one (or both, if your vehicle is so equipped) first.  I would start with the one at the converter outlet (or behind the converter).  The sensors have about a 50,000 mile life expectancy.