Volvo Repair: 2001 volvo v70xc, factory warranty, o2 sensor

Question
My v70 check engine light has gone off three times but I have only written the code down the last two times.  The second time the check engine light went on the code read "TWC-Cat. Conv. effic.-code."  The shop that pulled the code said that I have a bad O2 sensor.  He reset it and said to drive it and see if the light comes on again.  After a couple of days the light came on again and I took it in and the code was "ECM 201A thermostat fault."  The mechanic had no idea what this code meant.  Can you help any advise me what you think is going on with the car?

Answer
Bryan, that TWC code should be covered under factory warranty for 7 years so I would check with the dealer.  That thermostat code is for the ECT sensor.  Its the sensor on the thermostat housing.  Replace it as well as the thermostat.  Common problem on these cars.  Roger