You WANT the downstream sensor not to read as high or as low as the upstream. When you graph the sensor voltages on the scan tool, you should see the downstream sensor follow the upstream sensor, but the downstream sensor shouldn't move as far up and down from centre (0.45V) as the upstream. This is because a working converter will burn up excess air and fuel in the exhaust and "mask" the signal of the upstream sensor. Here is a crappy MS paint drawing I made to show you what things SHOULD look like:
Notice how the downsteam sensor follows the upstream, but it'* curve is much less pronouced? This is good. When scanning a vehicle with a non-functioning converter, you'll see that the two curves are almost exactly identical. In other words, the converter is doing nothing to mask the signal of the upstream sensor. It'* dead and nothing is going on in there. The PCM watches the O2 sensors for this similarity in the curves, and this is when it throws a catalyst efficiency code.
The catalyst efficiency code can also come up in some other situations. One is when you've got a misfire that'* dumping raw fuel (and oxygen) into the exhaust and confusing the O2 sensors. Another is when you've got a bad sensor. Watch their voltages on the scantool. The upstream should cycle back and forth between a high value (above 0.45V, ex 0.7V) to a low value (below 0.45V, ex 0.3V) and back again. It should do it quickly and constantly. A sensor that takes a long time to cycle is often called a "lazy" sensor. The downstream sensor should cycle too, but more slowly and as mentioned before, not as far from centre. You can make sure the sensors are reading properly with a *sort-of* quick test. Keep the car idling for this. First give the throttle a quick jab, right to the floor. This will cause the PCM to dump fuel and the A/F ratio will go rich for a moment. Both sensors should read high as soon as you do this, then go back to their normal cycling after a second. Once things have stabilized, do another test. Pull a vacuum line off to induce a lean condition and watch your voltages. Both sensors should read low. If one of your sensors doesn't respond as expected, or can't reach high voltages of over 0.8/low voltages of under 0.2, replace it.
Worn out sensors usually lose their high voltage capability first. Hence they loose the ability to tell the PCM it'* rich. This is why changing old O2 sensors sometimes makes a big improvement to fuel economy.