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I experienced something very similar with same symptoms, SC430 engine in limp mode. The issue was due to a microscopic air leak developed in the after market cold air intake system. I ordered a replacement Injen intake and it ran perfect thereafter. Maybe check for any loose hose clamps on intake?
They are dumping out behind the primary cats so hopefully aren't messing with the primary sensors. I did that so I wouldn't throw a CEL with the cutouts.
All O2 sensors need clean air on one side and exhaust on the other. If exhaust is getting to the clean air side, the O2 sensor will be sending crazy data the ECM does not expect and it will light up a ton of codes. Same thing if it gets water in the air vents. See the diagram below for how the thimble type works (this is the same construction as the secondary O2 sensors in our cars). The primary O2 is a planar type which works on a similar principle, but is constructed very differently.
The other possibility is twisted wires causing cross talk (the signal from an O2 sensor is measured in microamps, so it is very sensitive) or physical damage to the sensor(s) and wiring.
