Attaching a digital voltmeter inside the car to the wire and to ground showed that the oxygen sensor output never got above 250 mV, showing a lean condition. The car was compensating for this lean signal by running as rich as it possibly could. Removing the sensor was easy, and I attached an ohmmeter to the sensor and found it to be infinite ohms. At this time I did not know that this was normal, so I went to the auto parts store and asked for a new oxygen sensor. I had my ohmmeter with me, and tested the new sensor right there at the counter, under the apprehensive gaze of the counterperson.
Many mechanics think that an oxygen sensor can be damaged by an ohmmeter, but they are wrong.
I found the new sensor also had infinite ohms, but bought it anyway. Smooth as silk. Sensor output toggles up and down, the car runs better. It takes a couple of days before the car is running optimally. I hypothesize that the fuel-injection system has a group of mixture profiles for various operating conditions that are updated very slowly.