Yeah, if it was running ok before and then suddenly it starts reading 18%, that's a problem. It could be that the TPS itself is bad though typically those go bad by putting out low voltage not high= run lean.
Did you try disconnecting that downstream sensor? That will throw a code, but if it is the only thing that is the problem, then everything else should clear up. I'd also remove the O2 heater fuse for that circuit. This will also throw a code, but that's ok if it runs like that. At least you'll know where to start looking. Anything more than 1v is not right for an O2 sensor.
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