Cat efficiency low could be caused by quite a few things, Malfunction Indicator Light (MIL) eliminators are mainly used for when you remove the cats (either gutted or headers) and they just fool the system into thinking the cats are running right, the PCM checks the upper oxygen sensors for what is coming out of the engine to check AF ratio and other things, the bottom oxygen sensors are just a check to see what's coming out of the cat to confirm they are scrubbing the exhaust like they should.
The code your getting, could just be junk, first thing to do is clear it (either unplug negative battery cable for a minute, or erase it with a code scanner) and then just drive it to see if it comes back on. Since you're not pulling an O2 code as well, I wouldn't look at those yet, but it might be a good idea to do a tune up if you haven't in a while. A lot of times when that's the only code pulled though, it's a garbage code and once erased will not return.