Are you sure no one had the PCM electronics modded for the gutted converter. My sons SRT-4 Neon also has a gutted converter but the previous owner sent the PCM out to have the electronics modded so it wont set the Eng lite on. The downstream 02 sensor does more then most realize.

First it should switch much slower then the upstream if the cat converter is doing its job. If the downstream 02 switching matches the upstream then of course it shows the converter not working. Also if the downstream shows the car running to lean or rich by the downstream voltage then it can cause the PCM to change the switching point of the upstream 02. Basically if it see's its readings showing the car running a bit rich then the PCM can lower the switching point voltage to lets say .4 volts instead of the normal .5 volts. In doing that when the car is being driven lean by the PCM when it see's a voltage over .5 instead of it switching to drive it rich when it goes below .5 it will keep driving it lean until it goes below .4 now to help makeup for the car running on the rich side. And vise versa if the car runs to lean going by the downstream 02 voltage.

That's just two of the functions it has as the PCM uses the downstream 02 reading in some of its other functions which is to much to get into now. My first guess is someone had the PCM modified to not set the eng lite by the downstream 02 readings ? Ron