An 02 sensor usually sets a code for itself pretty quickly. It'll even set a code for itself sometimes when you can find absolutely nothing wrong with it's reading. Usually that particular code is a vaccuum elbow. Though on about four occasions I have had that particular code be followed by a front fan that acts strangely, and that IS generally the coolant temp sensor. Replaced the ECT sensor, no more running rich bank one code, no more strange fan behavior, and never even got a code for the ECT. Since you were there replacing the thermostat, you could have incidentally destroyed the magic.