Q: Rear O2 sensor range? on 2002 Toyota Camry

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My check engine light is on. Code is for cat. I'm trying to find out if the cat is bad or if one of the O2 sensors is bad.
(1) Answer
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The computer in your car compares the reading between the front and rear Oxygen Sensor to see if the Catalytic Converter is working efficiently. The sensors have a heater element in them to heat the sensor internally, the sensor must be kept at a high temperature to operate correctly, (if the element fails it will set a code related to heater circuit open). A lazy sensor can allow the computer to misinterpret the operating efficiency of the catalytic converter setting a low efficiency code. The sensor output need to be tested using an oscilloscope comparing the front and rear sensor pattern , the front sensor should show activity and the rear sensor pattern should be relatively flat in a properly operating catalytic converter. The catalytic converter should using a non contact thermometer show a temperature difference front to rear of 150 to 200 degrees F at 3000 engine RPM.