Toyota Repair: 07 Camry 4 cyl, oxygen sensor, trouble code

Question
engine light on, tested computer, indicates oxygen sensor bad . i talked to a parts guy he said about $200 , but could be one or several. How many are there ? Is there a test Toyota can do to pinpoint exactly which component is bad since it appears to be computer related and generated ? Can it harm other components and how serious is it ? I am about 4000 miles from my service dealer !

Answer
Yes there is a test that the dealer can do to determine which sensor is not working, it requires connecting a diagnostic scan tool to read the output voltages of the sensors, there are two sensors, one in front of the catalityc converter and one behind the converter, any competent repair shop should aqlso be able to do this but maybe I can save some time if you can tell me the diagnostic trouble code number that is in the computer now or was, when it was tested. I need a specific trouble code number.