Originally Posted by Dirty Biker
Can the downstream o2 sensor keep it from running in closed loop?
No, but the CEL can. If the downstream O2s pick up an emissions issue, it can send it into fits and alter the fuel and timing curves to prevent emissions.
Either replace the cat (expensive) or put some o2 sims in place of the rear O2 sensor. (slightly less expensive)
Or... the equally illegal route would be to have someone reflash your PCM to ignore the downstream O2s. (slightly less expensive)