The code that was set comes from when the oxygen sensor was not being seen at all. That happens when the voltage does not budge from 0.5V, which comes from the ECU and not the sensor. But your sensor is working well now. That is the behavior of a loose connection. Amarin makes one good point -- that the exhaust pipes provide the sensor's signal ground path back toward the ECU. That was the big reason car makers added a 4th wire soon after these cars were designed. If you have rust, loose exhaust pipe joints, the OBD may have caught the condition to set the 212 code. Then you warm it up and the metal makes better contact, or a bit of movement scrapes off the rust.
--
Art Benstein near Baltimore

Advice for the day: If you have a lot of tension and you get a headache, do what it says on the aspirin bottle: "Take two aspirin" and "Keep away from children."
(Author Unknown)