It is likely that your catalyst is contaminated, and is not storing oxygen, which is what the O2 sensor based catalyst monitor measures. It does this by comparing the front O2 sensor signal to the rear. If the catalyst is storing oxygen, the rear O2 sensor voltage should not switch very often (go from + to - voltage). If the rear is switching, it means that your catalyst is passing oxygen through-hence the loss of efficiency code.

Reasons for this vary. Burning too much oil can poison a catalyst. Some fuel, for example canada or maybe Mexico, can poison catalysts designed for the USA because of additives. If you had a dead cylinder for a while, and didn't fix it, the misfire could have fried the cat. Also, if you ran excessively rich or lean, you could have over-heated the cat and cooked it.

The good news is that there is (in the USA) a government mandated 80,000 mile warranty on the catalyst. The bad news is that if you are over 80,000 miles, you will need to spend ~$500 on a new catalyst. Which you probably should do, as you are polluting the air as long as you drive with a dead cat.