Autozone's analysis on my MPV's Check Engine Light says "Warm-up catalyst system (RH) efficiency below threshold".
What does that mean?

 

It means that the pre-cat that is hardest to reach (at the firewall) is operating at less than perfect efficiency. The downstream Oxygen sensor is active because of unburned fuel residue passing through the converter unconverted. It is a rather expensive repair -- even with aftermarket parts.

Other issues could be involved. If you have a leak in your exhaust manifold you can get this error even if your catalyst is good.