If you are not familiar with the OBDII (On Board Diagnostic generation II) systems used on 3rd generation and later Toyotas, you might want to read this reference before proceeding further in this article. As described in the reference article, the oxygen sensor in the exhaust system is the key "feedback" sensor for correcting and maintaining the proper fuel mixture. It measures the post-combustion gases to determine the actual air/fuel mixture of the prior combustion cycle and then, the ECU adjusts the fuel trim percentage on the next combustion cycle, to keep the fuel mixture at the optimum value.
As the oxygen sensor ages, its responsiveness begins to slow down. Since I have a BR-3 OBD-II scanner that can read the voltages generated by the O2 sensor, I thought I would be able to see some indication in the voltage signal that my O2 sensor was not providing optimum performance. However when I examined the O2 Sensor Output with the engine idling, it appeared to be pretty much the same as I had seen when the vehicle was new. Also, the error codes applicable to the O2 Sensor had not shown up and turned on the Check Engine light.
However, if the Check Engine Light were to turn on with one of the following O2 Sensor codes stored, you should follow the diagnostic procedure and replace the O2 sensor if so indicated in the diagnostic procedure.
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