What else causes Trouble code P0420?

I have a 1995 Toyota Tacoma rear drive 2.4 L and I had this problem since I purchased this vehicle used in 1999. I am now on my 4th catalytic converter and they recently cleaned both O2 sensors to no avail. I used an gas additive to clean the O2 and everything else inside and will work for 3 weeks and now I get the P0420 turn on every day. I read somewhere that this should have been a recall because of the throttle body design. Unless you want to use gas additives to clean your fuel or buy the real OEM catalytic converter.