Downstream Cat codes are set for signal low, signal high, low efficiency. Low signal is usually a broken harness wire that the ecu sees as 0v. If it's at .01-.2 then the wire is either corroded (high resistance), or the cat has lost its ability to cover CO (CO + O2 = CO2+) Efficiency failing code is set when the cat is unavailable to convert anything, and is passing the fluctuating o2 exhaust thru the cat. The downstream sensor starts looking like the upstream sensor When failed high, it will show either a) a short in the wire thus sending .5v back without any fluctuation or B) send a consistent result above .7 - an indication of low O2. O2 is produced on N conversion activities (and consumed on the CO conversion).