My van was getting progressively poor gas milage and losing power.  I suspect it was a bad o2 sensor. Just installed a new one today and haven't had a chance to check it out.  

From past experience, a bad o2 sensor can burn out the cat converter.  Running without the o2 sensor will do the same thing.  

My advice is to replace with the OEM part.  

Dick
87 Vanagon GL.  
Just under 197 k miles